Global access to water

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In 2004 about 3.5 billion people worldwide (54% of the global population) had access to piped water supply through house connections. Another 1.3 billion (20%) had access to an improved water source through other means than house, including standpipes, "water kiosks", protected springs and protected wells. Finally, more than 1 billion people (16%) did not have access to an improved water source, meaning that they have to revert to unprotected wells or springs, canals, lakes or rivers to fetch water. It should be noted that access to an improved source of water does not necessarily imply that it is safe to drink from that source.

Service quality

Many of the 3.5 billion people having access to piped water receive a poor or very poor quality of service, especially in developing countries where about 80% of the world population lives. Water supply service quality has many dimensions: continuity; water quality; pressure; and the degree of responsiveness of service providers to customer complaints.

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Continuity of supply

Continuity of water supply is taken for granted in most developed countries, but is a severe problem in many developing countries, where sometimes water is only provided for a few hours every day or a few days a week. It is estimated that about half of the population of developing countries receives water on an intermittent basis.

Water quality

Drinking water quality has a micro-biological and a physico-chemical dimension. There are thousands of parameters of water quality. In public water supply systems water should, at a minimum, be disinfected - most commonly through the use of chlorination or the use of ultra violet light - or it may need to undergo treatment, especially in the case of surface water. For more details please see the separate entries on water quality, water treatment and drinking water.

Water pressure

Water pressures vary in different locations of a distribution system. Water mains below the street may operate at higher pressures, with a pressure reducer located at each point where the water enters a building or a house. In poorly managed systems, water pressure can be so low as to result only in a trickle of water or so high that it leads to damage to plumbing fixtures and waste of water. Pressure in an urban water system is typically maintained either by a pressurized water tank serving an urban area, by pumping the water up into a tower and relying on gravity to maintain a constant pressure in the system or solely by pumps at the water treatment plant and repeater pumping stations.

Typical UK pressures are 4-5 bar for an urban supply. However, some people can get over 8 bars. A single iron main pipe may cross a deep valley, it will have the same nominal pressure, however each consumer will get a bit more or less because of the hydrostatic pressure (about 1 bar/10m height). So people at the bottom of a 100-foot hill will get about 3 bars more than those at the top.

The effective pressure also varies because of the supply resistance even for the same static pressure. An urban consumer may have 5 metres of 1/2" lead pipe running from the iron main, so the kitchen tap flow will be fairly unrestricted, so high flow. A rural consumer may have a kilometre of rusted and limed 3/4" iron pipe so their kitchen tap flow will be small.

For this reason the UK domestic water system has traditionally (prior to 1989) employed a "cistern feed" system, where the incoming supply is connected to the kitchen sink and also a header/storage tank in the attic. Water can dribble into this tank through a 1/2" lead pipe, plus ball valve, and then supply the house on 22 or 28 mm pipes. Gravity water has a small pressure (say 1/4 bar in the bathroom) but needs wide pipes allow higher flows. This is fine for baths and toilets but is frequently inadequate for showers. People install shower booster pumps to increase the pressure. For this reason urban houses are increasingly using mains pressure boilers (combies) which take a long time to fill a bath but suit the high back pressure of a shower.

Comparing the performance of water and sanitation service providers

Comparing the performance of water and sanitation service providers (utilities) is needed, because the sector offers limited scope for direct competition (natural monopoly). Firms operating in competitive markets are under constant pressure to out perform each other. Water utilities are often sheltered from this pressure, and it frequently shows: some utilities are on a sustained improvement track, but many others keep falling further behind best practice. Benchmarking the performance of utilities allows to simulate competition, establish realistic targets for improvement and create pressure to catch up with better utilities. Information on benchmarks for water and sanitation utilities is provided by the International Benchmarking Network for Water and Sanitation Utilities.[1]

Institutional responsibility and governance

A great variety of institutions have responsibilities in water supply. A basic distinction is between institutions responsible for policy and regulation on the one hand; and institutions in charge of providing services on the other hand.

Policy and regulation

Water supply policies and regulation are usually defined by one or several Ministries, in consultation with the legislative branch. In the United States the United States Environmental Protection Agency‎, whose administrator reports directly to the President, is responsible for water and sanitation policy and standard setting within the executive branch. In other countries responsibility for sector policy is entrusted to a Ministry of Environment (such as in Mexico and Colombia), to a Ministry of Health (such as in Panama, Honduras and Uruguay), a Ministry of Public Works (such as in Ecuador and Haiti), a Ministry of Economy (such as in German states) or a Ministry of Energy (such as in Iran). A few countries, such as Jordan and Bolivia, even have a Ministry of Water. Often several Ministries share responsibilities for water supply. In the European Union, important policy functions have been entrusted to the supranational level. Policy and regulatory functions include the setting of tariff rules and the approval of tariff increases; setting, monitoring and enforcing norms for quality of service and environmental protection; benchmarking the performance of service providers; and reforms in the structure of institutions responsible for service provision. The distinction between policy functions and regulatory functions is not always clear-cut. In some countries they are both entrusted to Ministries, but in others regulatory functions are entrusted to agencies that are separate from Ministries.

Regulatory agencies

Dozens of countries around the world have established regulatory agencies for infrastructure services, including often water supply and sanitation, in order to better protect consumers and to improve efficiency. Regulatory agencies can be entrusted with a variety of responsibilities, including in particular the approval of tariff increases and the management of sector information systems, including benchmarking systems. Sometimes they also have a mandate to settle complaints by consumers that have not been dealt with satisfactorily by service providers. These specialized entities are expected to be more competent and objective in regulating service providers than departments of government Ministries. Regulatory agencies are supposed to be autonomous from the executive branch of government, but in many countries have often not been able to exercise a great degree of autonomy. In the United States regulatory agencies for utilities have existed for almost a century at the level of states, and in Canada at the level of provinces. In both countries they cover several infrastructure sectors. In many US states they are called Public Utility Commissions. For England and Wales, a regulatory agency for water (OFWAT) was created as part of the privatization of the water industry in 1989. In many developing countries, water regulatory agencies were created during the 1990s in parallel with efforts at increasing private sector participation. (for more details on regulatory agencies in Latin America, for example, please see Water and sanitation in Latin America and the regional association of water regulatory agencies ADERASA [6])

Many countries do not have regulatory agencies for water. In these countries service providers are regulated directly by local government, or the national government. This is, for example, the case in the countries of continental Europe, in China and India.

For more information on utility regulation in the water sector see the body of knowledge on utility regulation [7] and the World Bank's knowledge base on the same topic at [8]

Service provision

Water supply service providers, which are often utilities, differ from each other in terms of their geographical coverage relative to administrative boundaries; their sectoral coverage; their ownership structure; and their governance arrangements.

Geographical coverage

Many water utilities provide services in a single city, town or municipality. However, in many countries municipalities have associated in regional or inter-municipal or multi-jurisdictional utilities to benefit from economies of scale. In the United States these can take the form of special-purpose districts which may have independent taxing authority. An example of a multi-jurisdictional water utility in the United States is WASA, a utility serving Washington, DC and various localities in the state of Maryland. Multi-jurisdictional utilities are also common in Germany, where they are known as "Zweckverbaende", in France and in Italy.

In some federal countries there are water service providers covering most or all cities and towns in an entire state, such as in all states of Brazil and some states in Mexico (see Water supply and sanitation in Mexico). In England and Wales water supply and sewerage is supplied almost entirely through ten regional companies. Some smaller countries, especially developed countries, have established service providers that cover the entire country or at least most of its cities and major towns. Such national service providers are especially prevalent in West Africa and Central America, but also exist, for example, in Tunisia, Jordan and Uruguay (see also water supply and sanitation in Uruguay). In rural areas, where about half the world population lives, water services are often not provided by utilities, but by community-based organizations which usually cover one or sometimes several villages.

Sector coverage

Some water utilities provide only water supply services, while sewerage is under the responsibility of a different entity. This is for example the case in Tunisia. However, in most cases water utilities also provide sewer and wastewater treatment services. In some cities or countries utilities also distribute electricity. In a few cases such multi-utilities also collect solid waste and provide local telephone services. An example of such an integrated utility can be found in the Colombian city of Medellín. Utilities that provide water, sanitation and electricity can be found in Frankfurt, Germany (Mainova), in Casablanca, Morocco and in Gabon in West Africa. Multi-utilities provide certain benefits such as common billing and the option to cross-subsidize water services with revenues from electricity sales, if permitted by law.

Ownership and governance arrangements

Water supply providers can be either public, private, mixed or cooperative. Most urban water supply services around the world are provided by public entities. But in most middle and low-income countries, these publicly-owned and managed water providers are usually very inefficient as a result of political interference, leading to over-staffing and low labour productivity. Ironically, the main losers from this institutional arrangement are the urban poor in these countries. Because they are not connected to the network, they end up paying far more per litre of water than do more well-off households connected to the network who benefit from the implicit subsidies that they receive from loss-making utilities. As Willem-Alexander, Prince of Orange (2002) stated, "The water crisis that is affecting so many people is mainly a crisis of governance - not of water scarcity." The introduction of cost-reflective tariffs together with cross-subsidisation between richer and poorer consumers is an essential governance reform in order to reduce the high levels of Unaccounted or Water (UAW) and to provide the finance needed to extend the network to those poorest households who remain unconnected. Partnership arrangements between the public and private sector can play an important role in order to achieve this objective [2]

Private sector participation

An estimated 10 percent of urban water supply is provided by private or mixed public-private companies, usually under concessions, leases or management contracts. Under these arrangements the public entity that is legally responsible for service provision delegates certain or all aspects of service provision to the private service provider for a period typically ranging from 4 to 30 years. The public entity continues to own the assets. These arrangements are common in France and in Spain. Only in few parts of the world water supply systems have been completely sold to the private sector (privatization), such as in England and Wales as well as in Chile. The largest private water companies in the world are Suez and Veolia Environnement from France; Aguas de Barcelona from Spain; and Thames Water from the UK, all of which are engaged internationally (see links to website of these companies below).

Governance arrangements

Governance arrangements for both public and private utilities can take many forms. Governance arrangements define the relationship between the service provider, its owners, its customers and regulatory entities. They determine the financial autonomy of the service provider and thus its ability to maintain its assets, expand services, attract and retain qualified staff, and ultimately to provide high-quality services. Key aspects of governance arrangements are the extent to which the entity in charge of providing services is insulated from arbitrary political intervention; and whether there is an explicit mandate and political will to allow the service provider to recover all or at least most of its costs through tariffs and retain these revenues. If water supply is the responsibility of a department that is integrated in the administration of a city, town or municipality, there is a risk that tariff revenues are diverted for other purposes. In some cases, there is also a risk that staff are appointed mainly on political grounds rather than based on their professional credentials. These risks are particularly high in developing countries. Municipal or inter-municipal utilities with a separate legal personality and budget as well as a certain extent of managerial autonomy can mitigate these risks.

Tariffs

Almost all service providers in the world charge tariffs to recover part of their costs. According to estimates by the World Bank the average (mean) global water tariff is US$ 0.53 per cubic meter. In developed countries the average tariff is US$ 1.04, while it is only U$ 0.11 in the poorest developing countries. The lowest tariffs in developing countries are found in South Asia (mean of US$ 0.09/m3), while the highest are found in Latin America (US$ 0.41/m3).[3] Few utilities do recover all their costs. According to the same World Bank study only 30% of utilities globally, and only 50% of utilities in developed countries, generate sufficient revenue to cover operation, maintenance and partial capital costs.

According to another study undertaken in 2006 by NUS Consulting, the average water and sewerage tariff in 14 mainly OECD countries excluding VAT varied between US$ 0.66 per cubic meter in the United States and the equivalent of US$ 2.25 per cubic meter in Denmark.[4] However, it should be noted that water consumption in the US is much higher than in Europe. Therefore, residential water bills may be very similar, even if the tariff per unit of consumption tends to be higher in Europe than in the US.

A typical family on the US East Coast paid between US$30 and US$70 per month for water and sewer services in 2005.[5]

In developing countries tariffs are usually much further from covering costs. Residential water bills for a typical consumption of 15 cubic meters per month vary between less than US$ 1 and US$ 12 per month.[6]

Water and sanitation tariffs, which are almost always billed together, can take many different forms. Where meters are installed, tariffs are typically volumetric (per usage), sometimes combined with a small monthly fixed charge. In the absence of meters, flat or fixed rates - which are independent of actual consumption - are being charged. In developed countries, tariffs are usually the same for different categories of users and for different levels of consumption.

In developing countries, are often characterized by cross-subsidies with the intent to make water more affordable for residential low-volume users that are assumed to be poor. For example, industrial and commercial users are often charged higher tariffs than public or residential users. Also, metered users are often charged higher tariffs for higher levels of consumption (increasing-block tariffs). However, cross-subsidies between residential users do not always reach their objective. Given the overall low level of water tariffs in developing countries even at higher levels of consumption, most consumption subsidies benefit the wealthier segments of society.[7] Also, high industrial and commercial tariffs can provide an incentive for these users to supply water from other sources than the utility (own wells, water tankers) and thus actually erode the utility's revenue base.

Metering

Metering of water supply is usually motivated by one or several of four objectives: First, it provides an incentive to conserve water which protects water resources (environmental objective). Second, it can postpone costly system expansion and saves energy and chemical costs (economic objective). Third, it allows a utility to better locate distribution losses (technical objective). Fourth, it allows to charge for water based on use, which is perceived by many as the fairest way to allocate the costs of water supply to users. Metering is considered good practice in water supply and is widespread in developed countries, except for the United Kingdom. In developing countries it is estimated that half of all urban water supply systems are metered and the tendency is increasing.

Water meters are read by one of several methods:

the water customer writes down the meter reading and mails in a postcard with this info to the water department;

the water customer writes down the meter reading and uses a phone dial-in system to transfer this info to the water department;

the water customer logs in to the website of the water supply company, enters the address, meter ID and meter readings [9]

a meter reader comes to the premise and enters the meter reading into a handheld computer;

the meter reading is echoed on a display unit mounted to the outside of the premise, where a meter reader records them;

a small radio is hooked up to the meter to automatically transmit readings to corresponding receivers in handheld computers, utility vehicles or distributed collectors

a small computer is hooked up to the meter that can either dial out or receive automated phone calls that give the reading to a central computer system.

Most cities are increasingly installing Automatic Meter Reading (AMR) systems to prevent fraud, to lower ever-increasing labor and liability costs and to improve customer service and satisfaction.

Costs and Financing

The cost of supplying water consists to a very large extent of fixed costs (capital costs and personnel costs) and only to a small extent of variable costs that depend on the amount of water consumed (mainly energy and chemicals). The full cost of supplying water in urban areas in developed countries is about US$1-2 per cubic meter depending on local costs and local water consumption levels. The cost of sanitation (sewerage and wastewater treatment) is another US$1-2 per cubic meter. These costs are somewhat lower in developing countries. Throughout the world, only part of these costs is usually billed to consumers, the remainder being financed through direct or indirect subsidies from local, regional or national governments (see section on tariffs).

Besides subsidies water supply investments are financed through internally generated revenues as well as through debt. Debt financing can take the form of credits from commercial Banks, credits from international financial institutions such as the World Bank and regional development banks (in the case of developing countries), and bonds (in the case of some developed countries and some upper middle-income countries).

History

Throughout history people have devised systems to make getting and using water more convenient. Early Rome had indoor plumbing, meaning a system of aqueducts and pipes that terminated in homes and at public wells and fountains for people to use. London water supply infrastructure developed over many centuries from early mediaeval conduits, through major 19th century treatment works built in response to cholera threats, to modern large scale reservoirs.

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed in 1910 by U.S. Army Major (later Brig. Gen.) Carl Rogers Darnall (1867-1941), Professor of Chemistry at the Army Medical School. Shortly thereafter, Major (later Col.) William J. L. Lyster (1869-1947) of the Army Medical Department used a solution of calcium hypochlorite in a linen bag to treat water. For many decades, Lyster's method remained the standard for U.S. ground forces in the field and in camps, implemented in the form of the familiar Lyster Bag (also spelled Lister Bag). Darnall's work became the basis for present day systems of municipal water 'purification'.

Standardization

International standards for water supply system are covered by International Classification of Standards (ICS) 91.140.60 [8].

Outbreaks of diseases due to contaminated water supply

In 1854, a cholera outbreak in London's Soho district was identified by Dr. John Snow as originating from contaminated water from the Broad street pump. This can be regarded as the founding event of the science of epidemiology.

In 1980, a hepatitis A surge due to the consumption of water from a feces-contaminated well, in Pennsylvania [9]

In 1987, a cryptosporidiosis outbreak is caused by the public water supply of which the filtration was contaminated, in western Georgia [10]

Fluoride intoxication in a long-term hemodialysis unit of university hospital due to the failure of a water deionization system [11]

In 1993, a fluoride poisoning outbreak resulting from overfeeding of fluoride, in Mississippi [12]

Footnotes

↑ Nickson, Andrew & Francey, Richard, Tapping the Market: The Challenge of Institutional Reform in the Urban Water Sector, 2003

↑ World Bank 2006: Water, Electricity and the Poor. Who Benefits from Utility Subsidies?, p. 21 [2] Data for 132 cities were assessed. The tariff is estimate for a consumption level of 15 cubic meters per month

↑ NUS Consulting 2005-2006 International Water Report & Cost Survey [3] The study covered Denmark, Germany, the UK, Belgium, France, The Netherlands, Italy, Finland, Australia, Spain, South Africa, Sweden, Canada and the US. The methodology for assessing tariffs may be different from the methodology of the World Bank study cited above. It should be noted that the report means by "costs" average tariffs and not the costs of the utility, which can be lower or higher than average tariffs

↑ quoted from a comparison of 24 utilities on the US East Coast in the 2005 Annual Report of DC WASA, p. 38 [4] The comparison refers to a consumption level of 25 cubic feet per quarter

From LoveToKnow 1911

WATER SUPPLY. This article is confined to the
collection and storage of water for domestic and industrial uses
and irrigation, and
its purification
on a large scale. The conveyance of water is dealt with in the
article Aqueduct.

Collecting Areas Surface Waters. - Any area, large or
small, of the earth's surface from any part of which, if the ground
were impermeable, water would flow by gravitation past any point in a natural
watercourse is commonly known in Europe as the " hydrographic basin " above that
point. In English it has been called indifferently the " catchment
basin," the " gathering ground," the " drainage area " and the " watershed." The latter
term, though originally equivalent to the German Wasserscheide-
" water-parting " - is perhaps least open to objection. The
water-parting is the line bounding such an area and separating it
from other watersheds. The banks
of a watercourse or sides of a valley are distinguished as the
right and left bank respectively, the spectator
being understood to be looking down the valley.

The surface of the earth is rarely impermeable, and the
structure of the rocks largely determines the direction of flow of
so much of the rainfall as sinks into the ground and is not
evaporated. Thus the figure and area of a surface watershed may not
be coincident with that of the corresponding underground watershed;
and the flow in any watercourse, especially from a small watershed,
may, by reason of underground flow from or into other watersheds,
be disproportionate to the area apparently drained by that
watercourse.

When no reservoir exists, the volume of continuous supply from
any watershed area Dry is evidently limited to the
minimum, or, so-called, extreme dry weather flow of the
stream draining it. This cannot be determined from the
rainfall; it entirely depends upon the power of the soil and rock
to store water in the particular
area under consideration, and to yield it continuously to the
stream by means of concentrated springs or diffused seepage.
Mountain areas of io,000 acres and upwards, largely covered with
moorland, upon nearly imper meable rocks with few water-bearing
fissures, yield in temperate climates, towards the end of the
driest seasons, and therefore solely from underground, between a
fifth and .a quarter of a cubic foot per second per 1000 acres.
Throughout the course of the river Severn, the head-waters of which
are chiefly supplied from such formations, this rate does not
materially change, even down to the city of Worcester, past which the discharge flows
from 1,256,000 acres. But in smaller areas, which on the average
are necessarily nearer to the waterparting, the limits are much
wider. and the rate of minimum discharge is generally smaller.

Thus, for example, on woo acres or less, it commonly falls to
onetenth of a cubic foot, and upon an upland Silurian area of 940 acres, giving no visible
sign of any peculiarity, the discharge fell, on the 21st of
September 1893, to one-thirty-fifth of a cubic foot per second per
woo acres. In this case, however, some of the water probably passed
through the beds and joints of
rocks to an adjoining valley lying at a lower level, and had both
streams been gauged the average would probably have been
considerably greater. The Thames at Teddington, fed largely from cretaceous
areas, fell during ten days in September 1898 (the artificial
abstractions for the supply of London being added) to about one-sixth of a
cubic foot, and since 1880 the discharge has occasionally fallen,
in each of six other cases, to about one-fifth of a cubic foot per
second per woo acres. Owing, however, to the very variable
permeability of the strata, the tributaries of the Thames, when
separately gauged in dry seasons, yield the most divergent results.
It may be taken as an axiom that
the variation of minimum discharges from their mean values
increases as the separate areas diminish. In the eastern and
south-eastern counties of England even greater variety of dry weather
flow prevails than in the west, and upon the chalk formations there are generally no surface
streams, except such as burst out after wet weather and form the
so-called " bournes." On the other hand, some rocks in mountain
districts, notably the granites, owing to the great quantity of
water stored in their numerous fissures or joints, commonly yield a
much higher proportion of so-called dry weather flow.

When, however, a reservoir is employed to equalize the flow
during and before the period of dry weather, the minimum flow
continuously available may be increased to a much higher figure,
depending upon the capacity of that reservoir in relation to the
mean flow of the stream supplying it. In such a case the first
essential in determining the yield is to ascertain the rainfall.
For this purpose, if there are no rain-gauges on the drainage area in question, an
estimate may be formed from numerous gaugings throughout the
country, most of which are published in British Rainfall,
initiated by the late Mr G. J. Symons, F.R.S., and now carried on
by Dr H. R. Mill. But except in the hands of those who have spent
years in such investigations, this method may lead to most incorrect
conclusions. If any observations exist upon the drainage area
itself they are commonly only from a single gauge, and this gauge, unless the area is very
level, may give results widely different from the mean fall on the
whole area. Unqualified reliance upon single gauges in the past has
been the cause of serious errors in the estimated relation between
rainfall and flow off the ground.

The uncertainties are illustrated by the following actual
example: A battery of
fourteen rain-gauges, in the same vertical plane, on ground having
the natural profile shown by
the section (fig. I), gave during three consecutive years the
respective falls shown by the height of the dotted lines above the
datum line. Thus on the average, gauge C recorded 20% more than
gauge D only ft. distant; while at C, in 1897, the rainfall was
actually 30% greater than at J only 560 ft. away. The greatly
varying distribution of rainfall over that length of 1600 ft. is
shown by the dotted lines measured upwards from the datum to have
been remarkably consistent in the three years; and its cause - the
path necessarily taken in a vertical plane by the prevailing winds
blowing from A towards N - after passing the steep bank at C D -
may be readily understood. Such examples show the importance of
placing any rain-gauge, so far as possible, upon a plane surface of
the earth - horizontal, or so inclined that, if produced,
especially in the direction of prevailing winds, it will cut the
mean levels of the area whose mean rainfall is intended to be
represented by that gauge. It has been commonly stated that
rainfall increases with the altitude. This is broadly true. A rain-cloud raised vertically upwards
expands, cools and tends to precipitate; but in the actual passage
of rain-clouds over the surface of the earth other influences are
at work. In fig. 2 the thick line A C D E F CH I
fs F' G. FIG. 2.

i

!0 1.

p H

p E f

'S ? /,, i // ?/ ,/ /?// / /

?/ ,D/?/?i,?',G.,

represents the profile of a vertical section crossing two ranges
of hills and one valley. The arrows indicate the directions of the
prevailing winds. At the extreme left the rain-clouds are thrown
up, and if this were all, they would precipitate a larger
proportion of the moisture Since the above was written, this work
has been taken over by the " British Rainfall Organization." FIG.
I.

\ they contained as the altitude increased. But until the clouds
rise above the hill there is an obvious countervailing tendency to
compression, and in
steep slopes this may reduce or entirely prevent precipitation
until the summit is reached, when a fall of pressure with commotion
must occur. Very high mountain ranges usually consist of many
ridges, among which rain-clouds are entangled in their ascent, and
in such cases precipitation towards the windward side of the main
range, though on the leeward sides of the minor ridges of which it
is formed, may occur to so large an extent that before the summit
is reached the clouds are exhausted or nearly so, and in this case
the total precipitation is less on the leeward than on the windward
side of the main range; but in the moderate heights of the United
Kingdom it more commonly happens from the causes explained that
precipitation is prevented or greatly retarded until the summit of
the ridge is reached. The following cause also contributes to the
latter effect. Imagine eleven raindrops A to K to fall
simultaneously and equi-distantly from the horizontal plane A M. A
strong wind is urging the drops from left to right. The drops A and
K may be readily conceived to be equally diverted by the wind, and
to fall near the tops of the two hills respectively. Not so drop C,
for directly the summit is passed the wind necessarily widens out
vertically and, having a greater space to fill, loses forward
velocity. It may even eddy backwards, as indicated by the curved
arrows, and it is no uncommon thing, in walking up a steep hill in
the contrary direction to the flight of the clouds, to find that
the rain is coming from behind. Much the same tendency exists with
respect to all drops between B and E, but at F the wind has begun
to accommodate itself to the new regime and to assume more regular
forward motion, and as J is approached, where vertical contraction
of the passage through which the wind must pass takes place, there
is an increasing tendency to lift the raindrops beyond their proper
limits. The general effect is that the rain falling from between G
and K is spread over a greater area of the earth G'K' than that
falling from the equal space between B and F, which reaches the
ground within the smaller area B'F'. From this cause also,
therefore, the leeward side of the valley receives more rain than
the windward side. In the United Kingdom the prevailing winds are
from the south-west. and some misapprehension has been caused by
the bare, but perfectly correct, statement that the general slope
towards the western coast is wetter than that towards the eastern.
Over the whole width of the country from coast to coast, or of the
Welsh mountain ranges only, this is so; but it is nevertheless true
that the leeward side of an individual valley or range of hills
generally receives more rain than the windward side. Successive abstraction of
raindrops as the rain-clouds pass over ridge after ridge causes a
gradually diminishing precipitation, but this is generally
insufficient to reverse the local conditions, which tend to the
contrary effect in individual ranges. The neglect of these facts
has led to many errors in estimating the mean rainfall on watershed
areas from the fall observed at gauges in particular parts of those
areas.

In the simplest case of a single mountain valley to be used for
the supply of an impounding reservoir, the rainfall should be known
at five points, three being in the axis of the valley, of which one
is near the point of intersection of that axis with the boundary of
the watershed. Then, in order to connect with these the effect of
the rightand left-hand slopes, there should be at least one gauge
on each side about the middle height, and approximatel y in a line
perpendicular to the axis of the valley passing through the central
gauge. The relative depths recorded in the several gauges depend
mainly upon the direction of the valley and steepness of the
bounding hills. The gauge in the bottom of the valley farthest from
the source will in a wide valley generally record the least
rainfall, and one of those on the south-west side, the highest.
Much will depend upon the judicious placing of the gauges. Each
gauge should have for io or 15 yds. around it an uninterrupted
plane fairly representing the general level or inclination, as the
case may be, of the ground for a much larger distance around it.
The earliest records of such gauges should be carefully examined,
and if any apparently anomalous result is obtained, the cause
should be traced, and when not found in the gauge itself, or in its
treatment, other gauges should be used to check it. The central
gauge is useful for correcting and checking the others, but in such
a perfectly simple case as the straight valley above assumed it may
be omitted in calculating the results, and if the other four gauges
are properly placed, the arithmetical mean of their results will
probably not differ widely from the true mean for the valley. But
such records carried on for a year or many years would afford no
knowledge of the worst conditions that could arise in longer
periods, were it not for the existence of much older gauges not far
distant and subject to somewhat similar conditions. The nearer such
long-period gauges are to the local gauges the more likely are
their records to rise and fall in the same proportion. The work of
the late Mr James Glaisher,F.R.S., of the late Mr G.J. Symons,
F.R.S., of the Meteorological Office and of the Royal
Meteorological Society, has resulted in the establishment of a vast
number of raingauges in different parts of the United Kingdom, and
it is generally, though not always, found that the mean rainfall
over a long period can be determined, for an area upon which the
actual fall is known only for a short period, by assigning to the
missing years of the shortperiod gauges, rainfalls bearing the same
proportion to those of corresponding years in the long-period
gauges that the rainfalls of the known years in the short-period
gauges bear to those of
corresponding years in the long-period gauges. In making such
comparisons, it is always desirable, if possible, to select as
standards longperiod gauges which are so situated that the
short-period district lies. between them. Where suitably placed
long-period gauges exist, and where care has been exercised in
ascertaining the authenticity of their, records and in making the
comparisons, the short records of the local gauges may be thus
carried back into the long periods with nearly correct results.

Rainfall is proverbially uncertain; but it would appear from the
most trustworthy records that at any given place the total rainfall
during any period of 50 years will be within i or 2% of the total
rainfall at the same place during any other period of 50 years,
while the records of any period of 25 years will generally be found
to fall within 32% of the mean of 50 years. It is equally
satisfactory to know that there is a nearly constant ratio on any
given area (exceeding perhaps 1000 acres) between the true mean
annual rainfall, the rainfall of the driest year, the two driest
consecutive years and any other groups of driest consecutive years.
Thus in any period of 50 years the driest year (not at an
individual gauge but upon such an area) will be about 63% of the
mean for the 50 years.

That in the two driest consecutive years will be about 75 °A of
the mean for the 50 years.

That in the three driest consecutive years will be about 80% of
the mean for the 50 years.

That in the four driest consecutive years will be about 83% of
themean for the 50 years.

That in the five driest consecutive years will be about 85% of
the mean for the 50 years.

That in the six driest consecutive years will be about 862% of
the mean for the 50 years.

Apart altogether from the variations of actual rainfall produced
by irregular surface levels, the very small area of a single
rain-gauge is subject to much greater variations in short periods
than can possibly occur over larger areas. If, therefore, instead
of regarding only the mean rainfall of several gauges over a series
of years, we compare the relative falls in short intervals of time
among gauges yielding the same general averages, the discrepancies
prove to be very great, and it follows that the maximum possible
intensity of discharge from different areas rapidly increases as
the size of the watershed decreases. Extreme cases of local
discharge are due to the phenomena known in America as " cloud-bursts," which occasionally
occur in Great Britain and result in
discharges, the intensities of which have rarely been recorded by
rain-gauges. The periods of such discharges are so short, their
positions so isolated and the areas affected so small, that we have
little or no exact knowledge concerning them, though their
disastrous results are well known. They do not directly affect. the
question of supply, but may very seriously affect the works from
which that supply is given.

Where in this article the term " evaporation " is used alone, it
is to be understood to include absorption by vegetation. Of the
total quantity of rainfall a very variable proportion is rapidly
absorbed or re-evaporated. Thus in the western mountain districts
of Great Britain, largely composed of nearly impermeable rocks more
Lion. or less
covered with pasture and moorland, the water evaporated and
absorbed by vegetation is from 13 to 15 in. out of a rainfall of 80
in., or from 16 to 19%, and is nearly constant down to about 60
in., where the proportion of loss is therefore from 22 to 25%. The
Severn down to Worcester, draining 1,256,000 acres of generally
flatter land largely of the same lithological character, gave in
the dry season from the 1st of July 1887 to the 30th of June 1888 a
loss of 17.93 in. upon a rainfall of 27.34 in. or about 66%; while
in the wet season, ist of July 1882 to the 30th of June 1883, the
loss was 21 09 in. upon a rainfall of 43.26 in., or only 49%. Upon
the Thames basin down to Teddington, having an area of 2,353,000
acres, the loss in the dry season from the ist of July 1890 to the
30th of June 1891 was 17.22 in. out of a rainfall of 21.62 in., or
79%; while in. the wet season, 1st of July 1888 to the 30th of June
1889, it was. 18.96 out of 29.22 in., or only 65%. In the eastern
counties the rainfall is lower and the evaporation approximately
the same as upon the Thames area, so that the percentage of loss.
is greater. But these are merely broad examples and averages. of
many still greater variations over smaller areas. They show
generally that, as the rainfall increases on any given area
evaporation increases, but not in the same proportion. Again, the
loss from a given rainfall depends greatly upon the previous
season. An inch falling in a
single day on a saturated mountain area will nearly all reach the
rivers, but if it falls during a drought seven-eighths may be lost
so far as the period of the drought is concerned. In such a case
most of the water is absorbed by the few upper inches of soil, only
to be re-evaporated during the next few days, and the small
proportion which sinks into the ground probably issues in springs
many months later. Thus the actual yield of rainfall to the streams
depends largely upon the mode of its time-distribution, and without
a knowledge of this it is impossible to anticipate the yield of a
particular rainfall. In estimating the evaporation to be deducted
from the rainfall for the purpose of determining the flow into a
reservoir, it is important to bear in mind that the loss from a
constant water surface is nearly one and a half times as great as
from the intermittently saturated land surface. Even neglecting the
isolated and local discharges due to excessive and generally
unrecorded rainfall, the variation in the discharge of all streams,
and especially of mountain streams, is very great. We have seen
that the average flow from mountain areas in Great Britain towards
the end of a dry season does not exceed one-fifth of a cubic foot
per second per 1000 acres. Adopting this general minimum as the
unit, we find that the flow from such areas up to about 5000 acres,
whose mean annual rainfall exceeds 50 in., may be expected
occasionally to reach 300 cub. ft., or 1500 such units; while from
similar areas of 20,000 or 30,000 acres with the same mean rainfall
the discharge sometimes reaches 1200 or 1300 such units. It is well
to compare these results with those obtained from much larger areas
but with lower mean rainfall. The Thames at Teddington has been
continuously gauged by the Thames Conservators since 1883, and the
Severn at Worcester by the writer, on behalf of the corporation of
Liverpool, during the io
years 1881 to 1890 inclusive. The highest flood, common to the two periods, was that which
occurred in the middle of February 1883. On that occasion the
Thames records gave a discharge of 7.6 cub. ft. per second per moo
acres, and the Severn records a discharge of 8.6 cub. ft. per
second per moo acres, or 38 and 43 respectively of the above units;
while in February 1881, before the Thames gaugings were commenced,
the Severn had risen to 47 of such units, and subsequently in May
1886 rose to 50 such units, though
the Thames about the same time only rose to 13. But in November
1894 the Thames rose to about 80 such units, and old records on the
Severn bridges show that
that river must on many occasions have risen to considerably over
100 units. In both these cases the natural maximum discharge is
somewhat diminished by the storage produced by artificial
canalization of the rivers.

These illustrations of the enormous variability of discharge
serve to explain what is popularly so little understood, namely,
the advantage which riparian owners, or other persons
Comperei nterested in a given stream, may derive from
works cation water. constructed primarily for the purpose
of diverting the water of that stream - it may be to a totally
different watershed - for the purposes of a town supply. Under
modern legislation no such abstraction of water is usually allowed,
even if limited to times of flood, except on condition of an augmentation of the
natural dry-weather flow, and this condition at once involves the
construction of a reservoir. The water supplied to the stream from
such a reservoir is known as " compensation water," and is generally a
first charge upon the works. This water is usually given as a
continuous and uniform flow, but in special cases, for the
convenience of millowners, as an intermittent one.' In the
manufacturing districts of Lancashire and Yorkshire it generally amounts to one-third
of the whole so-called " available supply." In Wales it is usually about one-fourth, and
elsewhere still less; but in any case it amounts to many times the
above unit of one-fifth of a cubic foot per second per 1000 acres.
Thus the benefit to the fisheries and to the riparian owners
generally is beyond all question; but the cost to the water
authority of conferring that benefit is also very great - commonly
(according to the proportion of the natural flow intended to be
rendered uniform) 20 to 35% of ' The volume of compensation water
is usually fixed as a given fraction of the so-called " available
supply " (which by a convention that has served its purpose well,
is understood to be the average flow of the stream during the three
consecutive driest years).

the whole expenditure upon the reservoir works. Down to the
middle of the 19th century, the proportioning of the size of a
reservoir to its work was a very rough operation. Yield of
There were few rainfall statistics, little was known stream
of the total loss by evaporation, and still less of its
with distribution over the different periods of dry and
reservoir. wet weather. Certain general principles have
since been laid down, and within the proper limits of their
application have proved excellent guides. In conformity with the
above-mentioned convention (by which compensation water is
determined as a certain fraction of the average flow during the
three driest consecutive years) the available supply or flow from a
given area is still understood to be the average annual rainfall
during those years, less the corresponding evaporation and
absorption by vegetation. But this is evidently only the case when
the reservoir impounding the water from such an area is of just
sufficient capacity to equalize that flow without possible
exhaustion in any one of the three summers. If the reservoir were
larger it might equalize the flow of the four or more driest
consecutive years, which would be somewhat greater than that of the
three; if smaller, we might only be able to count upon the average
of the flow of the two driest consecutive years, and there are many
reservoirs which will not yield continuously the average flow of
the stream even in the single driest year. With further experience
it has become obvious that very few reservoirs are capable of
equalizing the full flow of the three consecutive driest years, and
each engineer, in estimating the yield of such reservoirs, has
deducted from the quantity ascertained on the assumption that they
do so, a certain quantity representing, according to his judgment,
the overflow which in one or more of such years might be lost from
the reservoir. The actual size of the reservoir which would
certainly yield the assumed supply throughout the driest periods
has therefore been largely a matter of judgment. Empirical rules
have grown up assigning to each district, according to its average
rainfall, a particular number of days' supply, independently of any
inflow, as the contents of the reservoir necessary to secure a
given yield throughout the driest seasons. But any such
generalizations are dangerous and have frequently led to
disappointment and sometimes to needless expenditure. The exercise
of sound judgment in such matters
will always be necessary, but it is nevertheless important to
formulate, so far as possible, the conditions upon which that
judgment should be based. Thus in order to determine truly the
continuously available discharge of any stream, it is necessary to
know not only the mean flow of the stream, as represented by the
rainfall less the evaporation, but also the least favourable
distribution of that flow throughout any year.

The most trying time-distribution of which the author has had
experience in the United Kingdom, or which he has been able to
discover from a comparison of rainfalls upon nearly impermeable
areas exceeding woo acres, is graphically represented by the thick
irregular line in the left-hand half of fig. 3, where the total
flow for the driest year measures too on the vertical percentage
scale; the horizontal time scale being divided into calendar months.

The diagram applies to
ordinary areas suitable for reservoir construction and in which the
minimum flow of the stream reaches about one-fifth of a cubic foot
per second per moo acres. Correspondingly, the straight line a
a represents uniformly distributed supply, also cumulatively
recorded, of the same quantity of water over the same period. But,
apart from the diurnal fluctuations of consumption which may be equalized by local
" service reservoirs," uniform distribution of supply throughout
twelve months is rarely what we require; and to represent the
demand in most towns correctly, we should increase the angle of
this line to the horizontal during the summer and diminish it
during the winter months, as indicated by the dotted lines b
b. The most notable features of this particular diagram are as
follows: Up to the end of 59 days (to the 28th February) the rate
of flow is shown, by the greater steepness of the thick line, to be
greater than the mean for the year, and the surplus water - about i
i % of the flow during the year - must be stored; but during the
184 days between this and the end of the 243rd day (31st August)
the rate of flow is generally below the mean, while from that day
to the end of the year it is again for the most part above the
mean. Now, in order that a reservoir may enable the varying flow,
represented cumulatively by the irregular line, to be discharged in
a continuous and uniform flow to satisfy a demand represented
cumulatively by the straight line a a, its capacity must
be such that it will hold not only the II % surplus of the same
year, but that, on June loth, when this surplus has been used to
satisfy the demand, it will still contain the water c
d-19%stored from a previous year; otherwise between June 10th and
August 31st the reservoir will be empty and only the dry weather
flow of the stream will be available for supply. In short, if the
reservoir is to equalize the whole flow of this year, it must have
a capacity equal to the greatest deficiency c d of the
cumulative flow below the cumulative demand, plus the greatest
excess e f of the cumulative flow over the cumulative
demand. This capacity is represented by the height of the line
a'a' (drawn parallel to a a from the point of maximum
surplus f) vertically above the point of greatest
deficiency c, and equal, on the vertical scale, to the
difference between the height c = 48% and g= 78%
or 30% of the stream-flow during the driest year. A reservoir so
proportioned to the stream-flow with a proper addition to avoid
drawing off the bottom water, would probably be safe in Great
Britain in any year FIG. 3.

for a uniform demand equal to the cumulative stream-flow; or, if
it failed, that failure would be of very short duration, and would
probably only occur once in 50 years.

It may be at first sight objected that a case is assumed in
which there is no overflow before the reservoir begins to fall, and
therefore no such loss as generally occurs from that cause. This is
true, but it is only so because we have made our reservoir large
enough to contain in addition to its stock of 19%, at the beginning
of the year, all the surplus water that passes during the earlier
months in this driest year with its least favourable
time-distribution of flow. Experience shows, in fact, that if a
different distribution of the assumed rainfall occurs, that
distribution will not try the reservoir more severely while the
hitherto assumed uniform rate of demand is maintained. But, as
above stated, the time-distribution of demand is never quite
uniform. The particular drought shown on the diagram is the result
of an exceptionally early deficiency of rainfall which, in
conjunction with the variation of demand shown by the dotted line
b b, is the most trying condition. The reservoir begins to
fall at the end of February, and continues to do so with few and
short exceptions until the end of August, and it so happens that
about the end of August this dotted line, b b representing
actual cumulative demand, crosses the straight line a a of uniform
demand, so that the excess of demand, represented by the slope from
June to September, is balanced by the deficiency of demand,
represented by the flatter slope in the first five months, except
as regards the small quantity b e near the end of
February, which, not having been drawn off during January and
February, must overflow before the end of February. To avoid this
loss the II % is in this case to be increased by the small quantity
b e determined by examination of the variation of the
actual from a constant demand.

After the reservoir begins to fall - in this case at the end of
February - no ordinary change in the variation of demand can affect
the question, subject of course to the cumulative demand not
exceeding the reservoir yield for the assumed year of minimum
rainfall. In assuming a demand at the beginning of the year below
the mean, resulting in an overflow equal in this case to b
e at the end of February and increasing our reservoir to meet
it, we assume also that some additional supply to that reservoir
beyond the 11 % of the streamflow from the driest
year can be obtained from the previous year. In relation to this
supply from the previous year the most trying assumption is that
the rainfall of that year, together with that of the driest year,
will be the rainfall of the two driest consecutive years. We have
already seen that while the rainfall of the driest of 50 years is
about 63% of the mean, that of the driest two consecutive years is
about 15% of the mean. It follows, therefore, that the year
immediately preceding the driest cannot have a rainfall less than
about 87% of the mean. As the loss by evaporation is a deduction lying between a
constant figure and a direct proportional to the rainfall, we
should err on the safe side in assuming the flow in the second
driest year to be increased proportionally to the rainfall, or by
the difference between 63 and 87 equal to 24% of the mean of 50
years. This 24% of the 50 years' mean flow is 38% of the driest
year's flow in fig. 3, and is therefore much more than sufficient
to ensure the reservoir beginning the driest year with a stock
equal to the greatest deficiency-19% - of the cumulative flow of
that year beyond the cumulative demand.

But in determining the capacity of reservoirs intended to yield
a supply of water equal to the mean flow of two, three or more
years, the error, though on the safe side, caused by assuming the
evaporation to be proportional to the rainfall, is too great to be
neglected. The evaporation slightly increases as the rainfall
increases, but at nothing like so high a rate. Having determined
this evaporation for the second driest consecutive year and
deducted it from the rainfall - which, as above stated, cannot be
less than 87% of the mean of 50 years - we may, as shown on fig. 3,
extend our cumulative diagram of demand and flow into the reservoir
from one to two years.

From diagrams constructed upon these principles, the general
diagram (fig. 4) has been produced. To illustrate its use, assume
the case of a mean rainfall of 50 in., figured in the right-hand
column at the end of a curved line, and of 14 in. of evaporation
and absorption by vegetation as stated in the note on the diagram.
The ordinate to any point
upon this curved line then represents on the left-hand scale the
maximum continuous yield per day for each acre of drainage area,
from a reservoir whose capacity is equal to the corresponding abscissa. As an example,
assume that we can conveniently construct a reservoir to contain,
in addition to bottom water not to be used, 200,000 gallons for
each acre of the watershed above the point of interception by the
proposed dam. We find on the
left-hand scale of yield that the height of the ordinate drawn to
the 50-inch mean rainfall curve
from 200,000 on the capacity scale, is 1457 gallons per day per
acre; and the straight radial line, which cuts the point of
intersection of the curved line and the co-ordinates, tells us that
this reservoir will equalize the flow of the two driest consecutive
years. Similarly, if we wish to equalize the flow of the three
driest consecutive years we change the co-ordinates to the radial
line figured 3, and thus find that the available capacity of the
reservoir must be 276,000 gallons per acre, and that in
consideration of the additional expense of such a reservoir we
shall increase the daily yield to 1612 gallons per acre. In the
same manner it will be found that by means of a reservoir having an
available capacity of only 118,000 gallons per acre of the
watershed, we may with the same rainfall and evaporation secure a
daily supply of 1085 gallons per acre. In this case the left-hand
radial line passes through the point at which the coordinates meet,
showing that the reservoir will just equalize the flow of the
driest year. Similarly, the yield from any given reservoir, or the
capacity required for any yield, corresponding with any mean
rainfall from 30 to 100 in., and with the flow over any period,
from the driest year to the six or more consecutive driest years,
may be determined from the diagram.

N Capacity of Reservoir.

Yield of Reservoir.

n

L'

0.

?.

8 A

PE

0.

°

.ooao'i

D

.p II

a

? 3

a ?

$

a u

y

(1)

(2)

(3)

(4)

(5)

(6)

(7)

162,000

1475

2

256,000

158.0

58 0

1922

130.3

30.3

3

352,000

21 7.3

37.5

2108

142.9

9.7

4

416,000

256.8

18.2

2220

150.5

5.3

5

466,000

287.7

12.0

2294

155.5

3.3

6

504,000

31 1 1

8.1

2350

159.3

2.4

It is instructive to note the ratio of increase of reservoir
capacity and yield respectively for any given rainfall. Thus,
assuming a mean rainfall of 60 in. during 50 years, subject to
evaporation and absorption equal to 14 in. throughout the dry
period under consideration, we find from the diagram the following
quantities (in gallons per acre of drainage area) and corresponding
ratios: - On comparing columns 3 and 6 or 4 and 7 it appears that
so great is the increase required in the size of a reservoir in
relation to its increased yield, that only in the most favourable
places for reservoir construction, or under the most pressing need,
can it be worth while to go beyond the capacity necessary to render
uniform the flow of the two or three driest consecutive years.

It must be clearly understood that the diagram fig. 4 does not
relieve the reader from any exercise of judgment, except as regards
the net capacity of reservoirs when
the necessary data have been obtained. It is merely a geometrical
determination of the conditions necessarily consequent in England,
Scotland and Wales, upon a
given mean rainfall over many years, upon evaporation and
absorption in particular years (both of which he must judge or determine for himself),
and upon certain limiting variations of the rainfall, already
stated to be the result of numerous records maintained in Great
Britain for more than 50 years. It must also be remembered that the
total capacity of a reservoir must be greater than its net
available capacity, in order that in the driest seasons fish life may be maintained and no
foul water may be drawn off.

Applied to most parts of Ireland and some parts of Great Britain, the
diagram will give results rather unduly on the safe side, as the
extreme annual variations of rainfall are less than in most parts
of Great Britain. Throughout Europe the annual variations follow
nearly the same law as in Great Britain, but in some parts the
distribution of rainfall in a single year is often more trying. The
droughts are longer, and the rain, when it falls, especially along
the Mediterranean coast, is often concentrated into shorter
periods. Moreover, it often falls upon sun-heated rocks, thus
increasing the evaporation for the time; but gaugings made by the
writer in the northern Apennines indicate that this loss is more
than compensated by the greater rapidity of the fall and of the
consequent flow. In such regions, therefore, for reservoirs
equalizing the flow of 2 or more years, the capacity necessary does
not materially differ from that required in Great Britain. As the
tropics are approached, even in mountain districts, the
irregularities become greater, and occasionally the rainy season is
entirely absent for a single year, though the mean rainfall is
considerable.

We have hitherto dealt only with the collection and storage of
that portion of the rainfall which flows over the surface of nearly
impermeable areas. Upon such areas the Springs
loss by percolation into the ground, not retrieved in and
the form of springs above the point of interception may be
neglected, and the only loss to the stream is that already
considered of re-evaporation into the air and of absorption by vegetation. But the crust
of the earth varies from almost complete impermeability to almost
complete permeability. Among the sedimentary rocks we have, for
example, in the clay slates of the
Silurian formations, rocks no less cracked and fissured than
others, but generally quite impermeable by reason of the joints
being packed with the very fine clay resulting from the rubbing of
slate upon slate in the earth
movements to which the cracks are due. In the New Red Sandstone, the Greensand and the upper
Chalk, we find the opposite extremes; while the igneous rocks are
for the most part only permeable in virtue of the open fissures
they contain. Wherever, below the surface, there are pores or open
fissures, water derived from rainfall is (except in the rare cases
of displacement by gas) found at levels above the sea determined by
the resistance of solids to its passage towards some neighbouring
sea, lake or watercourse. Any such level is commonly known as the
level of saturation. The positions of springs are determined by
permeable depressions in the surface of the ground below the
general level of saturation, and frequently also by the holding up
of that level locally by comparatively impermeable strata,
sometimes combined with a fault
or a synclinal fold of the strata,
forming the more permeable portion into an underground basin or
channel lying within comparatively impermeable boundaries. At the
lower lips or at the most permeable parts of these basins or
channels such rainfall as does not flow over the surface, or is not
evaporated or absorbed by vegetation, and does not, while still
below ground reach the level of the sea, issues as springs, and is
the cause of the continued flow of rivers and streams during
prolonged droughts. The average volume in dry weather, of such
flow, generally reduced to terms of the fraction of a cubic foot
per second, per thousand acres of the contributing area, is
commonly known in water engineering as the " dry weather flow " and
its volume at the end of the dry season as the " extreme dry
weather flow." Perennial springs of large volume rarely occur in
Great Britain at a sufficient height to afford supplies by
gravitation; but from the limestones of Italy and many other parts of the world very
considerable volumes issue far above the sea-level, and are thus
available, without pumping, for the supply of distant towns. On a
small scale, however, springs are fairly distributed over the
United Kingdom, for there are no formations, except perhaps blown
sand, which do not vary greatly in
their resistance to the percolation of water, and therefore tend to
produce overflow from underground at some points above the valley
levels. But even the rural populations have generally found surface
springs insufficiently constant for their use and have adopted the
obvious remedy of sinking wells.
Hence, throughout the world we find the shallow well still very
common in rural districts. The shallow well, however, rarely
supplies enough water for more than a few houses, and being
commonly situated near to those houses the water is often seriously
polluted. Deep wells owe their comparative immunity from pollution to the circumstances
that the larger quantity of water yielded renders it worth while to
pump that water and convey it by
pipes from comparatively unpolluted areas; and that any impurities
in the water must have passed through a considerable depth, and by
far the larger part of them through a great length of filtering
material, and must have taken so long a time to reach the well that
their organic character has disappeared. The principal
water-bearing formations, utilized in Great Britain by means of
deep wells, are the Chalk and the New Red Sandstone. The Upper and
Middle Chalk are permeable almost through their mass. They hold
water like a sponge, but part with it under pressure to fissures by
which they are intersected, and, in the case of the Upper Chalk, to
ducts following beds of flints. A well sunk in these formations
without striking any fissure or water-bearing flintbed, receives water only at a very
slow rate; but if, on the other hand, it strikes one or more of the
natural water-ways, the quantity of water capable of being drawn
from it will be greatly increased.

It is a notable peculiarity of the Upper and Middle Chalk
formations that below their present valleys the underground water
passes more freely than elsewhere. This is explained by the fact
that the Chalk fissures are almost invariably rounded and enlarged
by the erosion of carbonic acid carried from the surface by
the water passing through them. These fissures take the place of
the streams in an impermeable area, and those beneath the valleys
must obviously be called upon to discharge more water from the
surface, and thus be brought in contact with more carbonic acid, than similar fissures
elsewhere. Hence the best position for a well in the Chalk is
generally that over which, if the strata were impermeable, the
largest quantity of surface water would flow. The Lower Chalk
formation is for the most part impermeable, though it contains many
ruptures and dislocations or smashes, in the interstices of which
large bodies of water, received from the Upper and Middle Chalk,
may be naturally stored, or which may merely form passages for
water derived from the Upper Chalk. Thus despite the impermeability
of its mass large springs are occasionally found to issue from the
Lower Chalk. A striking example is that known as Lydden Spout,
under Abbot's Cliff, near Dover.
In practice it is usual in chalk formations to imitate artificially
the action of such underground watercourses, by driving from the well small tunnels, or " adits
" as they are called, below the water-level, to intercept fissures
and water-bearing beds, and thus to extend the collecting area.

Next in importance to the Chalk formations as a source of
underground water supply comes the Trias or New Red Sandstone,
consisting in Great Britain of two main divisions, the Keuper above and the Bunter below. With the exception
of the Red Marls forming the upper part of the Keuper, most of the
New Red Sandstone is permeable, and some parts contain, when
saturated, even more water than solid chalk; but, just as in the
case of the chalk, a well or borehole in the sandstone yields very
little water unless it strikes a fissure; hence, in New Red
Sandstone, also, it is a common thing to form underground chambers
or adits in search of additional fissures, and sometimes to sink
many vertical boreholes with the same object in view.

As the formation approaches the condition of pure sand, the
water-bearing property of any given mass increases, but the
difficulty of drawing water from it without admixture Wells
is of sand also increases. In sand below water there are,
sand. of course, no open fissures, and even if adits could
be usefully employed, the cost of constructing and lining them
through the loose sand would be prohibitive. The well itself must
be lined; and its yield is therefore confined to such water as can
be drawn through the sides or the bottom of the lining without
setting up a sufficient velocity to cause any sand to flow with the
water. Hence it arises that, in sand formations, only shallow wells
or small boreholes are commonly found. Imagine for a moment that
the sand grains were by any means rendered immobile without change
in the permeability of their interspaces; we could then dispense
with the iron or brickwork lining of the well; but as there
would still be no cracks or fissures to extend the area of
percolating water exposed to the open well, the yield would be very
small. Obviously, it must be very much smaller when the lining
necessary to hold up loose sand is used. Uncemented brickwork, or
perforated ironwork, are xxvill. 13 a the usual materials employed
for lining the well and holding up the sand, and the quantity of
water drawn is kept below the comparatively small quantity
necessary to produce a velocity, through the joints or orifices,
capable of disturbing the sand. The rate of increase of velocity
towards any isolated aperture through which water passes into the
side of a well sunk in a deep bed of sand is, in the neighbourhood
of that aperture, inversely proportional to the square of the
distance therefrom. Thus, the velocity across a little hemisphere
of sand only z in. radius
covering a i-in. orifice in the lining is more than 1000 times the
mean velocity of the same water approaching the orifice radially
when 16 in. therefrom. This illustration gives some idea of the
enormous increase of yield of such a well, if, by any means, we can
get rid of the frictional sand, even from Artificial
within the 16 in. radius. We cannot do this, but of
happily the grains in a sand formation differ very widely in
diameter, and if, from the interstices between the larger grains in
the neighbourhood of an orifice, we can remove the finer grains,
the resistance to flow of water is at once enormously reduced. This
was for the first time successfully done in a well, constructed by
the Biggleswade
Water Board in 1902, and now supplying water over a large area of
North Bedfordshire. This well, 10 ft. diameter,
was sunk through about 110 ft. of surface soil, glacial drift and impermeable gault clay and thence passed for a
further depth of 70 ft. into the Lower Greensand formation, the
outcrop of which, emerging on the south-eastern shore of the Wash, passes
south-westwards, and in Bedfordshire attains a thickness exceeding
250 ft. The formation is probably more or less permeable
throughout; it consists largely of loose sand and takes the general
south-easterly dip of British
strata. The Biggleswade well was sunk by processes better known in
connexion with the sinking of mine shafts and foundations of
bridges across the deep sands or gravels of bays, estuaries and
great rivers. Its full capacity has not been ascertained; it much
exceeds the present pumping power, and is probably greater than
that of any other single well unassisted by adits or boreholes.
This result is mainly due to the reduction of frictional resistance
to the passage of water through the sand in the immediate
neighbourhood of the well, by washing out the finer particles of
sand and leaving only the coarser particles. For this purpose the
lower 45 ft. of the cast-iron cylinders forming the well was
provided with about 660 small orifices lined with gun-metal tubes
or rings, each armed with numerous thicknesses of copper wire gauze, and temporarily closed with screwed plugs.
On the removal of any plug, this wire gauze prevented the sand from
flowing with the water into the well; but while the finer particles
of sand remained in the neighbourhood of the orifice, the flow of
water through the contracted area was very small. To remove this
obstruction the water was pumped out while the plugs kept the
orifices closed. A flexible pipe,
brought down from a steamboiler above, was then connected
with any opened orifice. This pipe was provided, close to the
orifice, with a three-way cock, by means of which the steam might
be first discharged into the sand, and the current between the cock
and the well then suddenly reversed and diverted into the well. The
effect of thus alternately forcing high-pressure steam among the
sand, and of discharging high-pressure water contained in the sand
into the well, is to break up any cohesion of the sand, and to
allow all the finer particles in the neighbourhood of the orifice
to rush out with the water through the wire gauze into the well.
This process, in effect, leaves each orifice surrounded by a
hemisphere of coarse sand across which the water flows with
comparative freedom from a larger hemisphere where the
corresponding velocity is very slow, and where the presence of
finer and more obstructive particles is therefore unimportant. Many
orifices through which water at first only dribbled were thus
caused to discharge water with great force, and entirely free from
sand, against the opposite side of the well, while the general
result was to increase the inflow of water many times, and to
entirely prevent the intrusion of sand. Where, however, a firm rock
of any kind is encountered, the yield of a well (under a given head
of water) can only be increased by enlargement.

of the main well in depth or diameter, or by boreholes or adits.
No rule as to the adoption of any one of these courses can be laid
down, nor is it possible, without examination of each particular
case, to decide whether it is better to attempt to increase the
yield of the well or to construct an additional well some distance
away. By lowering the head of water in any well which draws its
supply from porous rock, the yield is always temporarily increased.
Every well has its own particular level of water while steady
pumping at a given rate is going on, and if that level is lowered
by harder pumping, it may take months, or even years, for the water
in the interstices of the rock to accommodate itself to the new
conditions; but the permanent yield after such lowering will always
be less than the quantity capable of being pumped shortly after the
change. We have hitherto supposed the pumps for drawing the water
to have been placed in the well at such a level as to be
accessible, while the suction pipe only is below water. Pumps,
however, may be (and have been) placed deep down in boreholes, so
that water may be pumped from much greater depths. By this means
the head of pressure in the boreholes tending to hold the water
back in the rock is reduced, and the supply consequently increased;
but when the cost of maintenance is included, the increased supply
from the adoption of this method rarely justifies expectations.
When the water has been drawn down by pumping to a lower level its
passage through the sandstone or chalk in the neighbourhood of the
borehole is further resisted by the smaller length of borehole
below the water; and there are many instances in which repeated
lowering and increased pumping, both from wells and boreholes, have
had the result of reducing the water available, after a few years,
nearly to the original quantity. One other method - the use of the
so-called " air-lift " - should be mentioned. This ingenious device originated in America. The
object attained by the air-lift is precisely the same as that
attained by putting a pump some distance down a borehole; but
instead of the head being reduced by means of the pump, it is
reduced by mixing the water with air. A pipe is passed down the
borehole to the desired depth, and connected with air-compressors
at the surface. The compressors being set to work, the air is
caused to issue from the lower end of the pipe and to mix in fine
bubbles with the rising column of water, sometimes several hundred
feet in height. The weight of the column of water, or rather of
water and air mixed, is thus greatly reduced. The method will
therefore always increase the yield for the time, and it may do so
permanently, though to a very much smaller extent than at first;
but its economy must always be less than that of direct
pumping.

In considering the principles of well supplies it is important
to bear the following facts in mind. The crust of the earth, so far
as it is permeable and above the sea-level, receives from rainfall
its supply of fresh water. That supply, so far as it is not
evaporated or absorbed by vegetation, passes away by the streams or
rivers, or sinks into the ground. If the strata were uniformly
porous the water would lie in the rock at different depths below
the surface according to the previous quantity and distribution of
the rainfall. It would slowly, but constantly, percolate downwards
and towards the sea, and would ooze out at or below the sea-level, rarely
regaining the earth's surface earlier except in deep valleys.
Precisely the same thing happens in the actual crust of the earth,
except that, in the formations usually met with, the strata are so
irregularly permeable that no such uniform percolation occurs, and
most of the water, instead of oozing out near the sea-level, meets
with obstructions which cause it to issue, sometimes below the
sea-level and sometimes above it, in the form of concentrated
springs. After prolonged and heavy rainfall the upper boundary of
the sub-soil water is, except in high ground, nearly coincident
with the surface. After prolonged droughts it still retains more or
less the same figure as the surface, but at lower depths and always
with less pronounced differences of level.

Sedimentary rocks, formed below the sea or salt lagoons, must originally have contained salt
water in their interstices.

On the upheaval of such rocks above the sea-level, fresh water
from rainfall began to flow over their exposed surfaces, and, so
far as the strata were permeable, to lie in their interstices upon
the salt water. The weight of the water original salt
water above the sea-level, and of the fresh below water so
superimposed upon it, caused an overflow towards the sea. A hill,
as it were, of fresh water rested in the interstices of the rock
upon the salt water, and continuing to press downwards, forced out
the salt water even below the level of the sea. Subject to the rock
being porous this process would be continued until the greater
column of the lighter fresh water balanced the smaller head of sea
water. It would conceivably take but a small fraction of the period
that has in most cases elapsed since such upheavals occurred for
the salt water to be thus displaced by fresh water, and for the
condition to be attained as regards saturation with fresh water, in
which with few exceptions we now find the porous portions of the
earth's crust wherever the rainfall exceeds the evaporation. There
are cases, however, as in the valley of the Jordan, where the
ground is actually below the sea-level, and where, as the total
evaporation is equal to or exceeds the rainfall, the lake surfaces
also are below the sea-level. Thus, if there is any percolation
between the Mediterranean and the Dead Sea, it must be towards the latter. There
are cases also where sedimentary rocks, formed below the sea or
salt lagoons, are almost impermeable: thus the salt deposited in
parts of the Upper Keuper of the New Red Sandstone, is protected by
the red marls of the formation, and has never been washed out. It
is now worked as an important industry in 'Cheshire.

Perhaps the most instructive cases of nearly uniform percolation
in nature are those which occur in some islands or peninsulas
formed wholly of sea sand. Here water is maintained above the
sea-level by the annual rainfall, and may be drawn off by wells or
borings. On such an island, in the centre of which a borehole is
put down, brackish water may be reached far below the sea-level;
the salt water forming a saucer, as it were, in which the fresh
water lies. Such a saltwater saucer of fresh water is maintained
full to overflowing by the rainfall, and owing to the frictional
resistance of the sand and to capillary action and the fact that a
given column of fresh water is balanced by a shorter column of sea
water, the fresh water never sinks to the mean sea-level unless
artificially abstracted.

Although such uniformly permeable sand is rarely met with in
great masses, it is useful to consider in greater detail so simple
a case. Let the irregular thick line in fig. 5 be the section of a
circular island a mile and a quarter in diameter, of uniformly
permeable sand.

The mean sea-level is shown by the horizontal line aa, dotted where it passes through
the land, and the natural mean level of saturation bb,
above the sea-level, by a curved dot and dash line. The water,
contained in the interstices of the sand above the mean sea-level,
would (except in so far as a film, coating the sand particles, is
held up by capillary attraction) gradually sink to the sea-level if
there were no rainfall. The resistance to its passage through the
sand is, however, sufficiently great to prevent this from occurring
while percolation of annual rainfall takes place.

Missing imageWatersupply-3.jpg

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Hence we may suppose that a condition has been attained in which
the denser salt water below and around the saucer CC (greatly
exaggerated in vertical scale) balances the less dense, but deeper
Salb Water v sand,. fresh water within it. Next suppose a
well to be sunk in the middle of the island, and a certain quantity
of water to be drawn therefrom daily. For small supplies such a
well may be perfectly successful; but however small the quantity
drawn, it must obviously have the effect of diminishing the volume
of fresh water, which contributes to the maintenance of the level
of saturation above the sea-level; and with further pumping the
fresh water would be so far drawn upon that the mean level of
saturation would sink, first to a curved figure - a cone of depression - such as that represented by
the new level of saturation dd, and later to the figure
represented by the lines ee, in which the level of
saturation has everywhere been drawn below the mean sea-level.
Before this stage the converse process begins, the reduced column
of fresh water is no longer capable of balancing the sea water in
the sand, inflow occurs at c and e, resulting
finally in the well water becoming saline. The figure, in this case
of uniform percolation, assumed by the water in the neighbourhood
of a deep well is a surface of revolution, and, however irregular
the percolation and the consequent shape of the figure, it is
commonly, but somewhat incorrectly, called the " cone of
depression. " It cannot have straight, or approximately straight,
sides in any vertical plane, but in nature is an exceedingly
irregular figure drawn about curves - not unlike those in fig. 5.
In this case, as in that of a level plane of uniformly porous sand,
the vertical section of the figure is tangential to the vertical
well and to the natural level of the subsoil water.

The importance of this illustration is to be found elsewhere
than in islands, or peninsulas, or in uniformly porous sand. Where
the strata are not uniformly porous, they may resist the passage of
water from the direction of the sea or they may assist it; and
round the whole coast of England, in the Magnesian limestone to the northeast,
in the Chalk and Greensand to the east and south, and in the New
Red Sandstone to the west, the number of wells which have been
abandoned as sources of potable supply, owing to the percolation of
sea water, is very great. Perhaps the first important cases
occurred in the earlier part of the 19th century on the Lancashire
shore of the Merseyestuary, where, one after
another, deep wells in the New Red Sandstone had to be abandoned
for most purposes. On the opposite side, in the Cheshire peninsula,
the total quantity of water drawn has been much less, but even here
serious warnings have been received. In 1895 the single well then
supplying Eastbourne
was almost suddenly rendered unfit for use, and few years pass
without some similar occurrence of a more or less serious kind. The
remarkable suddenness with which such changes are brought about is
not to be wondered at when the true cause is considered. The action
of sandstone in filtering salt waters was investigated in 1878 by
Dr Isaac Roberts, F.R.S., who showed that when
salt water was allowed to percolate blocks of sandstone, the
effluent was at first nearly fresh, the salt being filtered out and
crystallized for the most part near the surface of ingress to the sandstone. As
the process continued the salt-saturated layer, incapable of
further effective filtration, grew in thickness downwards, until in
the process of time it filled the whole mass of sandstone. But
before this was accomplished the filtration of the effluent became
defective, and brackish water was received, which rapidly increased
nearly to the saltness of the inflow. Into such blocks, charged
with salt crystals and thoroughly dried, fresh water was then
passed, and precisely the converse process took place. A thickness
of only 12 in. of Bunter sandstone proved at first to be capable of
removing more than 80% of the chlorides from sea water; but, after
the slow passage of only o 6 gallon through 1 cub. ft. of stone, the
proportion removed fell to 8.51%. The general lesson to be learned from these facts is, that
if the purity of the water of any well not far removed from the sea
is to be maintained, that water must not be pumped down much below
the sea-level. In short, the quantity of water drawn must in no
case be allowed to exceed the quantity capable of being supplied to
the well through the medium of the surrounding soil and rock, by
rain falling upon the surface of the land. If it exceeds this, the
stock of fresh water held in the interstices of the rock, and
capable of flowing towards the well, must disappear; and the
deficit between the supply and demand can only be made up by water
filtering from the sea and reaching the well at first quite free
from salt, but sooner river water whatever. Thus natural or
artificial surfaces which are completely permeable to rainfall may
become almost impermeable when protected by surface water from
drought and frost, and from
earth-worms, vegetation and artificial
disturbance. The cause of this choking of the pores is precisely
the same as that described below in the case of sand filters. But
in order that the action may be complete the initial resistance to
percolation of water at every part of the soil must be such that
the motion of the water through it shall be insufficient to disturb
the water-borne mineral and organic particles lodged on the surface
or in the interstices of the soil. If, therefore, a reservoir so
formed survives the first few years without serious leakage, it is
not likely, in the absence of artificial disturbance, to succumb
owing to leakage at a later period. Hence, as the survival of the
fittest, there are many artificial waters, with low dams consisting
exclusively of earth - and sometimes very sandy earth -
satisfactorily performing their functions with no visible leakage.
But it is never advisable to rely upon this action, where, as in
the case of a reservoir for water supply, large portions of
naturally permeable bottom are liable to be uncovered and exposed
to the weather.

The most important dams are those which close the outlets of
existing valleys, but a dam may be wholly below ground, and
according to the commoner method of construction in Great Britain,
sufficient) im ermeable construc- ?
y p tion. rising ground is not met with at the
intended boundary of a reservoir, a trench is cut along such
portion, and carried down to rock or such other formation as, in
the engineer's opinion, forms a sufficiently impermeable sheet beneath the whole surface to
be covered with water. Into this trench socalled " puddled clay,"
that is, clay rendered plastic by kneading with water, is filled
and thoroughly worked with special tools, and trodden in layers. In
this manner an underground compartment is formed, the bottom of
which is natural, and the sides partly natural and partly
artificial, both offering high resistance to the passage of water.
Above ground, if the water level is to be higher than the natural
boundary, the same puddle walls or cores are carried up to the
required level, and are supported as they rise by embankments of
earth on either side.

Fig. 6 is a typical section of a low dam of this class,
impounding water upon gravel
overlying impermeable clay. In such a structure the whole attention
as regards water-tightness should be concentrated upon the puddle
wall or core. When, as may happen in dry seasons, the puddle wall
remains long above the water level, it parts with moisture and
contracts. It is essential that this contraction shall not proceed
to such an extent as may possibly produce cracking. Drying is
retarded, and the contraction due to a given degree of drying is
greatly reduced, by the presence of sand and small stones among the
clay. Nearly all clays, notably those from the Glacial deposits,
naturally contain sand and stones, 40 to 50% by weight of which is
not too much if uniformly distributed an y 1 if the clay is
otherwise good. But in the lower parts of the trench, where the
Overflow level or later in a condition unfit for use.

Dams Any well-made earthen embankment of moderate height, and of such
thickness and uniformity of construction as to ensure freedom from
excessive percolation at any point, will in the course of time
become almost impermeable to surface water standing against it; and
when permeable rocks are covered with many feet of soil, the
leakage through such soil from standing water newly placed above it
generally diminishes rapidly, and in process of time often ceases
entirely. Even the beds of sluggish rivers flowing over porous
strata generally become so impermeable that excavations made in
their neighbourhood, though freely collecting the subsoil water,
receive no FIG. 6. - Section of Typical Low Earth Embankment in
Flat Plain.

clay can never become dry, plasticity and ductility are, for
reasons to be explained below, the first consideration, and there
the proportion of grit should be lower. The resistance of clay to
percolation by water depends chiefly upon the density of the clay, while that density is
rapidly reduced if the clay is permitted to absorb water. Thus, if
dry clay is prevented from expanding, and one side be sub j ected
to water pressure while the other side is held up by a completely
porous medium, the percolation will be exceedingly small; but if
the pressure preventing the expansion is reduced the clay will
swell, and the percolation will increase. On the restoration of the
pressure, the density will be again increased by the reduction of
the water-filled interstices, and the percolation will be
correspondingly checked. Hence the extreme importance in high dams
with clay cores of loading the clay well for some time before water
pressure is brought against it. If this is done, the largest
possible quantity of clay will be slowly but surely forced into any
space, and, being prevented from expanding, it will be unable
subsequently to absorb more water. The percolation will then be
very small, and the risk of disintegration will be reduced to a
minimum. The embankments on either side of the puddle wall are
merely to support the puddle and to keep it moist above the ground
level when the reservoir is low. They may be quite permeable, but
to prevent undue settlement and distortion they must, like the
puddle, be well consolidated. In order to prevent a tendency to
slip, due to sudden and partial changes of satura tion, the outer
embankment should always be permeable, and well drained at the base
except close to the puddle. The less permeable materials should be
confined to the inner parts of the embankments; this is especially
important in the case of the inner embankment in order that, when
the water level falls, they may remain moist without becoming
liable to slip. The inner slope should be protected from the action
of waves by so-called " hand-pitching," consisting of
roughlysquared stonework, bedded upon a layer of broken stone to
prevent local disturbance of the embankment by action of the water
between the joints of the larger stones.

In mountain valleys, rock or shale, commonly the most
impermeable materials met with in such positions, are sometimes not
reached till considerable depths are attained. There are several
cases in Great Britain where it has been necessary to carry down
the puddle trench to about zoo ft. below the surface of the ground
vertically above those parts. The highest dams of this class in the
British islands impound water to a level of about
IIo ft. above the bottom of the valley. Such great
works have generally been well constructed, and there are many
which after fifty years of use are perfectly sound and water-tight,
and afford no evidence of deterioration. On the other hand, the
partial or total failure of smaller dams of this description, to
retain the reservoir water, has been much more common in the past
than is generally supposed. Throughout Great Britain there are
still many reservoirs, with earthen dams, which cannot safely be
filled; and others which, after remaining for years in this
condition, have been repaired. From such cases and their successful
repair valuable experience of the causes of failure may be
derived.

Missing imageWatersupply-5.jpg

Missing imageWatersupply-6.jpg

Missing imageWatersupply-7.jpg

Most of these causes are perfectly well understood by
experienced engineers, but instances of by malconstruction
of recent date are still met with., A few such cases will now be
mentioned. The base of a puddle trench is often found to have been
placed upon rock, perfectly sound in itself, but having joints
which are not impermeable. The loss of water by leakage through
such joints or fissures below the puddle wall may or may not be a
serious matter in itself; but if at any point there is sufficient
movement of water across the base of the trench to produce the
slightest erosion of the clay above it, that movement almost
invariably increases. The finer particles of clay in the line of
the joint are washed away, while the sandy particles, which nearly
all natural clays contain, remain behind and form a constantly
deepening porous vein of sand crossing the base of the puddle.
Percolation toe and concrete t through this sand
is thus added to the original leakage. Having passed through the
puddle core the leaking water sometimes rises to the surface of the
ground, producing a visibly turbid spring. As erosion proceeds, the
contraction of the space from which the clay is washed continues,
chiefly by the sinking down of the clay above the sand. Thus the
permeable vein grows vertically rather than horizontally, and
ultimately assumes the form of a thin vertical sheet traversing the
puddle wall, often diagonally in plan, and having a thickness which
has varied in different cases from a few inches to a couple of feet
or more, of almost clean sand rising to an observed height of 30 or
40 ft., and only arrested in its upward growth by the necessary
lowering of the reservoir water to avoid serious danger. The
settlement of the plastic clay above the eroded portion soon
produces a surface depression at the top of the embankment over or
FIG. 7. - Earth Embankment, with stone TOP Bank Level % / // /
/ / i i FIG. 8. - Leakage due to improperly formed discharge
culvert-through puddle wall of reservoir.

nearly over the leakage, and thus sometimes gives the first
warning of impending danger. It is not always possible to prevent
any leakage whatever through the strata below the bottom or beyond
the ends of the trench, but it is always possible to render such
leakage entirely harmless to the work above it, and to carry the
water by relief-pipes to visible points at the lower toe of the
dam. Wherever the base of a puddle wall cannot be worked into a
continuous bed of clay or shale, or tied into a groove cut in sound
rock free from water-hearing
fissures, the safest course is to base it on an artificial material
at once impermeable and incapable of erosion, interposed between
the rock and the puddled clay. Water-tight concrete is a suitable
material for the purpose; it need not be made so thick as the
puddle core, and is therefore sometimes used with considerable
advantage in lieu of the puddle for the whole depth below ground.
In fig. 7 a case is shown to be so treated. Obviously, the junction
between the puddle and the concrete might have been made at any
lower level.

However well the work may be done, the lower part of a mass of
puddled clay invariably settles into a denser mass when weighted
with the clay above. If, therefore, one part is held up, by
unyielding rock for example, while an adjoining part has no support
but the clay beneath it, a fracture - not unlike a geological fault
- must result. Fig. 8 is a part longitudinal section through the
puddle wall of an earthen embankment. The puddle wall is crossed by
a pedestal of concrete
carry- - 3 ing the brick discharge cul v ert. The
puddle at a was originally held up by the flat head of this
pedestal; not so the puddle at b, which under the
superincumbent weight settled down and produced the fault
bc, accompanied with a shearing or tangential strain or, less probably, with
actual fracture in the direction bd. Serious leakage at
once began between c and b and washed out the
clay, particle by particle, but did not wash out the sand
associated with it, which remained rench.

behind in the crevice. The clay roof, rather than the walls of
this crevice of sand, gave way and pressed down to fill the
vacancy, and the leakage worked up along the weakened plane of
tangential strain bd. On the appearance of serious leakage
the overflow level of the water originally at of was
lowered for safety to gh; and for many years the reservoir
was worked with its general level much below gh. The
sand-filled vein, several inches in width, was found, on taking out
the puddle, to have terminated near the highest level to which the
water was allowed to rise, but not to have worked downwards. There
can be little doubt that the puddle at the right-hand angle
j was also strained, but not to the point of rupture, as
owing to the rise of the sandstone base there was comparatively
little room for settlement on that side. In repairing this work the
perfectly safe form shown by the dotted lines ka, kj was
substituted for the flat surface aj, and this alone, if
originally adopted, would have prevented dangerous shearing
strains. As an additional precaution, however, deep tongues of
concrete like --- j { those in fig. 7 were built in the rock
throughout the length of the trench, and carried up the sides and
over the top of the ped estal. The puddle was then replaced, and
remains sensibly watertight. The lesson taught by fig. 8 applies
also to the ends of puddle walls where they abut against steep
faces of rock. Unless such faces are so far below the surface of
the puddle, and so related to the lower parts of the trench, that
no tension, and consequent tendency to separation of the puddle
from the rock, can possibly take place, and unless abundant time is
given, before the reservoir is charged, for the settlement and
compression of the puddle to be completed, leakage with disastrous
results may occur.

In other cases leakage and failure have arisen from allowing a
part of the rock bottom or end of a puddle trench to overhang, as
in fig. 9. Here the straining of the original horizontal puddle in
settling down is indicated in a purposely exaggerated way by the
curved lines. There is considerable distortion of the clay,
resulting from combined shearing and tensile stress, above each of
the steps of rock, and reaching its maximum at and above the
highest rise ab, where it has proved sufficient to produce
a dangerous line of weakness ac, the tension at a either
causing actual rupture, or such increased porosity as to permit of
percolation capable of keeping open the wound. In such cases as are shown in figs. 8 and
9 the growth of the sand vein is not vertical, but inclined towards
the plane of maximum shearing strain. Fig. 9 also illustrates a
weak place at b where the clay either never pressed hard
against the overhanging rock or has actually drawn away therefrom
in the process of settling towards the lower part to the left. When
it is considered that a parting of the clay, sufficient to allow
the thinnest film of water to pass, may start the formation of a
vein of porous sand in the manner above explained, it will be
readily seen how great must be the attention to details, in
unpleasant places below ground, and below the water level of the
surrounding area, if safety is to be secured. In cases like fig. 9
the rock should always be cut away to a slope, such as that shown
in fig. io.

If no considerable difference of water-pressure had been allowed
between the two sides of the puddle trench in figs. 8 or 9 until
the clay had ceased to settle
down, it is probable that the interstices, at first formed between
the puddle and the concrete or rock, would have been sufficiently
filled to prevent injurious percolation at any future time. Hence
it is always a safe precaution to afford plenty of time for such
settlement before a reservoir is charged with water. But to all
such precautions should be added the use of concrete or brickwork
tongues running longitudinally at the bottom of the trench, such as
those shown at a higher level in fig. 7.

In addition to defects arising out of the condition or figure of
the rock or of artificial work upon which the puddle clay rests,
the puddle wall itself is often defective. The original material
may have been perfectly satisfactory, but if, for example, in
puddle the progress of the work a stream of water is
allowed to flow across it, fine clay is sometimes washed away, and
the gravel or sand associated with it left to a sufficient extent
to permit of future percolation. Unless such places are carefully
dug out or re-puddled before the work of filling is resumed, the
percolation may increase along the vertical plane where it is
greatest, by the erosion and falling in of the clay roof, as in the
other cases cited. Two instances probably originating in some such
cause are shown in fig. i I in the relative
positions in which they were found, and carefully measured, as the
puddle was removed from a crippled reservoir dam. These fissures
are in vertical planes stretching entirely across the puddle
trench, and reaching in one case, aa, nearly to the highest level
at which the reservoir had been worked for seventeen years after
the leakage had been discovered. The larger and older of these veins was 441 ft. high, of which 14
ft. was above the original ground level, and it is interesting to
note that this portion, owing probably to easier access for the
water from the reservoir and reduced compression of the puddle, was
much wider than below. The little vein to the left marked
bb, about 31 ft. deep, is curious. It looks like the
beginning of success of an effort made by a slight percolation
during the whole life of the reservoir to increase itself
materially by erosion.

FIG. I I. - Vertical Vein of Leakage.

There is no reason to believe that the initial cause of such a
leakage could be developed except during construction, and it is
certain that once begun it must increase. Only a knowledge of the
great loss of capital that has resulted from abortive reservoir
construction justifies this notice of defects which can always be
avoided, and are too often the direct result, not of design, but of
parsimony in providing during the execution of such works, and
especially below ground, a sufficiency of intelligent, experienced
and conscientious supervision.

In some cases, as, for example, when a high earthen embankment
crosses a gorge, and there is
plenty of stone to be had, it is desirable to place the outer bank
upon a toe or platform of
rubble stonework, as in fig.
7, by which means the height of the earthen portion is reduced and
complete drainage secured. But here again great care must be
exercised in the packing and consolidation of the stones, which
will otherwise crack and settle.

As with many other engineering works, the tendency to slipping
either of the sides of the valley or of the reservoir embankment
itself has often given trouble, and has sometimes led to serious
disaster.

Missing imageWatersupply-8.jpg

Missing imageWatersupply-9.jpg

FIG. 9. - Overhanging Rock Leakage.

._ ----- 1 ?? P U D D L E /? .

Missing imageWatersupply-10.jpg

FIG. io. - Proper Figure for Rock Slope.

This, however, is a kind of failure not always attributable to
want of proper supervision during construction, but rather to
improper choice of the site, or treatment of the case, by those
primarily responsible.

water-tight to begin with, the alternate immersion and exposure to air and sunshine promotes expansion
and contraction, and induces rapid disintegration, leakage and
decay. Such an expedient may be justified by the doubtful future of
mining centres, but would be
out of the question for permanent water supply. Riveted sheets of
steel have
been occasionally used, and, where bedded in a sufficient thickness
of concrete, with success. At the East CanonCreek
dam, Utah, the height of which is
about 6r ft. above the stream, the trench below ground was filled
with concrete much in the usual way, while above ground the
water-tight diaphragm
consists of a riveted steel plate varying in thickness from in. to
3 3 6 - in. This steel septum was protected on
either side by a thin wall of asphaltic concrete supported by
rubble stone embankments, and owing to irregular settling of 'the
embankments became greatly distorted, apparently, however, without
causing leakage. Asphalt,
whether a natural product or artificially obtained, as, for
example, in some chemical manufactures, is a most useful material
if properly employed in connexion with reservoir dams. Under sudden
impact it is brittle, and has a conchoidal fracture like glass; but under continued pressure
it has the properties of a viscous fluid. The rate of flow is
largely dependent upon the proportion of bitumen it contains, and is of course retarded
by mixing it with sand and stone to form what is commonly called
asphalt concrete. But given time, all such compounds, if they
contain enough bitumen to render them water-tight, appear to settle
down even at ordinary temperatures as heavy viscous fluids,
retaining their fluidity permanently if not exposed to the air.
Thus they not only penetrate all cavities in an exceedingly
intrusive manner, but exert pressures in all directions, which,
owing to the density of the asphalt, are more than 40 greater than
would be produced by a corresponding depth of water. From the
neglect of these considerations numerous failures have
occurred.

Elsewhere, a simple concrete or masonry wall or core has been used above as
well as below ground, being carried up between embankments either
of earth or rubble stone. This construction has received its
highest development in America. On the Titicus, a tributary of the
Croton river, an earthen dam was completed in 1895, with a concrete
core wall zoo ft. high - almost wholly above the original ground
level, which is said to be impermeable; but other dams of the same
system, with core walls of less than ioo ft. in height, are
apparently in their present condition not impermeable. Reservoir
No. 4 of the Boston
waterworks, completed in 1885, has a concrete core wall. The
embankment is 1800 ft. long and 60 ft. high. The core wall is about
8 ft. thick at the bottom and 4 ft. thick at the top, and in the
middle of the valley nearly ioo ft. in height. At irregular
intervals of 150 ft. or more buttresses 3 ft. wide and 1 ft. thick
break the continuity on the water side. That this work has been
regarded as successful is shown by the fact that Reservoir No. 6 of
the same waterworks was subsequently constructed and completed in
1894 with a similar core wall. There is no serious difficulty in so
constructing walls of this kind as to be practically water-tight
while they remain unbroken; but owing to the settlement of the
earthen embankments and the changing level of saturation they are
undoubtedly subject to irregular stresses which cannot be
calculated, and under which, speaking generally, plastic materials
are much safer. In Great Britain masonry or concrete core walls
have been generally confined to positions below ground. Thus
placed, no serious strains are caused either by changes of
temperature or of moisture or by movements of the lateral supports,
and with proper ingredients and care a very thin wall wholly below
ground may be made watertight.

The next class of dam to be considered is that in which the
structure as a whole is so bound together that, with certain
reservations, it may be considered as a monolith subject chiefly to
the overturning tendency of waterpressure resisted by the weight of
the structure itself and the supporting pressure of the foundation.
Masonry dams are, for the most part, merely retaining walls of
exceptional size, in which the overturning pressure is water. If
such a dam is sufficiently strong, and is built upon sound and
moderately rough rock, it will always be incapable of sliding.
Assuming also that it is incapable of crushing under its own weight
and the pressure of the water, it must, in order to fail entirely,
turn over on its outer toe, or upon the outer face at some higher
level. It may do this in virtue of horizontal water-pressure alone,
or of such pressure combined with upward pressure from intrusive
water at its base or in any higher horizontal plane. Assume first,
however, that there is no uplift from intrusive water. As the
pressure of water is nil at the surface and increases in
direct proportion to the depth, the overturning moment is as the cube of the depth; and the only
figure which has a moment of resistance due to gravity, varying
also as the cube of its depth, is a triangle. The form of stability having the
least sectional area is therefore a triangle. It is obvious that
the angles at the base of such a hypothetical dam must depend upon
the relation between its density and that of the water. It can be
shown, for example, that for masonry having a density of 3, water
being 1, the figure of minimum section is a right-angled triangle,
with the water against its vertical face; while for a greater
density the water face must lean towards the water, and for a less
density away from the water, so that the water may lie upon it. For
the sections of masonry dams actually used in practice, if designed
on the condition that the centre of all vertical pressures when the
reservoir is full shall be, as hereafter provided, at two-thirds
the width of the base from the inner toe, the least sectional area
for a density of 2 also has a vertical water face. As the density
of the heaviest rocks is only 3, that of a masonry dam must be
below 3, and in practice such works if well constructed vary from
2.2 to 2.6. For these densities, the deviation of the water face
from the vertical in the figure of least sectional area is,
however, so trifling that, so far as this consideration is
concerned, it may be neglected.

If the right-angled triangle abc, fig. 12, be a profile
i ft. thick of a monolithic dam, subject to the pressure of water
against its vertical side to the full depth ab= d in feet,
the horizontal _ eL 2 pressure of water against the
section of the dam, inI creasing uniformly with the depth, is
properly represented by the isosceles right-angled triangle
abe, in which be is the maximum water-pressure
due to the Cent full depth d, while the
area 2 abe = d is t h e total hor12 d3 6 If x be
the width of the base, and p the density of the masonry,
the weight of the masonry in terms of a cubic foot of water will be
acting at its centre of gravity g, situated at 3x from the
outer toe, and the moment of resistance to overturning on the outer
toe, p x 2 d (2) In countries where good clay or retentive
earth cannot be obtained, numerous alternative expedients have been
adopted with more or less success. In the mining districts of
America, for example, where timber is cheap, rough stone
embankments have been lined on the water face with timber to form
the water-tight septum. In such zontalpressure against the e dam,
generally stated in F IG. 12. - Diagram of Right-Angled cubic feet
of water, acting g at one-third its depth above Triangle Dam.

Missing imageWatersupply-11.jpg

d2 the base. Then a is the resultant horizontal pressure with an
over turning moment of (I) Equating the moment of resistance (2) to
the overturning moment (I), we have pxzd =d3 3 6 and x
=?2 p ..

(3) That is to say, for such a monolith to be on the point of
overturning under the horizontal pressure due to the full depth of
water, its base must be equal to that depth divided by the square
root of twice the density of the monolith. For a density of 2.5 the
base would therefore be 44.7% of the height.

We have now to consider what are the necessary factors of
safety, and the modes of their application. In the first place, it
is out of of the question to allow the water to rise to
the vertex a Factors . of such a masonry triangle. A
minimum thickness must safety be adopted to give substance
to the upper part; and where the dam is not used as a weir it must necessarily rise several
feet above the water, and may in either event have to carry a
roadway. Moreover, considerable mass is required to reduce the
internal strains caused by changes of temperature. In the next
place, it is necessary to confine the pressure, at every point of
the masonry, to an intensity which will give a sufficient factor of
safety against crushing. The upper part of the dam having been
designed in the light of these conditions, the whole process of
completing the design is simple enough when certain hypotheses have
been adopted, though somewhat laborious in its more obvious form.
It is clear that the greatest crushing pressure must occur, either,
with the reservoir empty, near the lower part of the water face
ab. or with the reservoir full, near the lower part of the
outer face ac. The principles hitherto adopted in
designing masonry dams, in which the moment of resistance depends
upon the figure and weight of the masonry, involve certain
assumptions, which, although not quite true, have proved useful and
harmless, and are so convenient that they may be continued with due
regard to the modifications which recent investigations have
suggested. One such assumption is that, if the dam is well built,
the intensity of vertical pressure will (neglecting local
irregularities) vary nearly uniformly from face to face along any
horizontal plane. Thus, to take the simplest case, if abce
(fig. 13) represents a rectangular mass already designed for the
superstructure FIG. 13. - Factor of Safety Diagram.

of the dam, and g its centre of gravity, the centre of
pressure upon the base will be vertically under g, that
is, at the centre of the base, and the load will be properly
represented by the rectangle bfgc, of which the area
represents the total load and the uniform depth of its uniform
intensity. At this high part of the structure the intensity of
pressure will of course be much less than its permissible
intensity. If now we assume the water to have a depth d
above the base, the total water pressure represented by the
triangle kbh will have its centre at d/3 from the
base, and by the parallelogram of forces, assuming the density of
the masonry to be 2.5, we find that the centre of pressure upon the
base bc is shifted from the centre of the base to a point
i nearer to the outer toe c, and adopting our
assumption of uniformly varying intensity of stress, the
rectangular diagram of pressures will thus be distorted from the
figure bfgc to the figure of equal area bjlc,
having its centre o vertically under the point at which
the resultant of all the forces cuts the base bc. For any
lower level the same treatment may, step by step, be adopted, until
the maximum intensity of pressure cl exceeds the assumed
permissible maximum, or the centre of pressure reaches an assigned
distance from the outer toe c, when the base must be
widened until the maximum intensity of pressure or the centre of
pressure, as the case may be, is brought within the prescribed
limit. The resultant profile is of the kind shown in fig. 14.

Having thus determined the outer profile under the conditions
hitherto assumed, it must be similarly ascertained that the water
face is everywhere cap able of resisting the vertical pressure of
the masonry when the reservoir is empty, and the base of each
compartment must be widened if necessary in that direction also.
Hence in dams above Too ft. in height, further adjustment of the outer
profile may be required by reason of the deviation of the inner
profile from the vertical. The effect of this process is to give a
series of points in the horizontal planes at which the resultants
of all forces above those planes respectively cut FIG. 14. -
Diagram showing lines of the planes. Curved pressure in Masonry
Dam. lines, as dotted in fig. 14, drawn through these points give
the centre of pressure, for the reservoir full and empty
respectively, at any horizontal plane. These general principles
were recognized by Messrs Graeff and Delocre of the Ponts et
Chaussees, and about the year 1866 were put into practice in the
Furens dam near St Etienne. In 1871 the late Professor Rankine,
F.R.S., whose remarkable perception of the practical fitness or
unfitness of purely theoretical deductions gives his writings
exceptional value, received from Major Tulloch, R.E., on behalf of
the municipality
of Bombay, a request to
consider the subject generally, and with special reference to very
high dams, such as have since been constructed in India. Rankine pointed out that before the
vertical pressure reached the maximum pressure permissible, the
pressure tangential to the slope might do so. Thus conditions of
stress are conceivable in which the maximum would be tangential to
the slope or nearly so, and would therefore increase the vertical
stress in proportion to the cosecant squared of the slope. It is
very doubtful whether this pressure is ever reached, but such a
limit rather than that of the vertical stress must be considered
when the height of a dam demands it. Next, Rankine pointed out
that, in a structure exposed to the overturning action of forces
which fluctuate in amount and direction, there should be no
appreciable tension at any point of the masonry. But there is a
still more important reason why this condition should be strictly
adhered to as regards the inner face. We have hitherto considered
only the horizontal overturning pressure of the water; but
if from originally defective construction, or from the absence of
vertical pressure due to weight of masonry towards the water edge
of any horizontal bed, as at ab in fig. 14, water intrudes
beneath that part of the masonry more readily than it can obtain egress along bc, or in
any other direction towards the outer face, we shall have the
uplifting and overturning pressure due to the full depth of water
in the reservoir over the width ab added to the horizontal
pressure, in which case all our previous calculations would be
futile. The condition, therefore, that there shall be no tension is
important as an element of design; but when we come to
construction, we must be careful also that no part of the wall
shall be less permeable than the water face. In fig. 13 we have
seen that the varying depth of the area bjlc approximately
represents the varying distribution of the vertical stress. If,
therefore, the centre of that became so far removed to the right as
to make j coincident with b, the diagram of
stresses would become the triangle j'l'c', and the vertical
pressure at the inner face would be nil. This will evidently happen
when the centre of pressure i' is two-thirds from the inner toe
b and one-third from the outer toec'; and if we displace the centre of pressure still
further to the right, the condition that the centre of figure of
the diagram shall be vertically under that centre of pressure can
only be fulfilled by allowing the point j to cross
the base to j" thus giving a negative pressure or tension
at the inner toe. Hence it follows that on the assumption of
uniformly varying stress the line of pressures, when the reservoir
is full, should not at any horizontal plane fall outside the middle
third of the width of that plane.

Rankine in his report adopted the prudent course of taking as
the safe limits certain pressures to which, at that time, such
structures were known to be subject. Thus for the inner face he
took, as the limiting vertical pressure, 320 ft. of water, or
nearly tons per sq. ft.. and for the outer face 250 ft. of
water, or about 7 tons per sq. ft.

Missing imageWatersupply-12.jpg

Missing imageWatersupply-13.jpg

For simplicity of calculation Rankine chose logarithmic curves
for both the inner and outer faces, and they fit very well with the
conditions. With one exception, however - the Beetaloo dam in Australia 110 ft. high -
there are no practical examples of dams with logarithmically curved
faces.

After Rankine, a French engineer, Bouvier, gave the ratio of the
maximum stress in a dam to the maximum vertical stress as 1 to the
cosine squared of the angle between the vertical and the resultant
which, in dams of the usual form, is about as 13 is to 9.

During the last few years attention has been directed to the
stresses - including shearing stresses - on planes other than
horizontal. M. Levy contributed
various papers on the subject which will be found in the
Comptes rendus de l'Academie des Sciences (1895 and 1898)
and in the Annales des Ponts et Chaussees (1897). He
investigated the problem by means of the general differential
equations of static equilibrium for dams of triangular and
rectangular form considered as isotropic elastic solids. In one of
these papers Levy formulated the requirement now generally adopted
in France that the vertical
pressure at the upstream end of any joint, calculated by the law of
uniformly varying stress, should not be less than that of the water
pressure at the level of that joint in order to prevent intrusive
water getting into the structure.

These researches were followed by those of Messrs L. W.
Atcherley and Karl Pearson, F.R.S.,1 and by an approximate
graphical treatment by Dr W. C. Unwin, F.R.S. 2 Dr Unwin took two
horizontal planes, one close above the other, and calculated the
vertical stresses on each by the law of uniformly varying stresses.
Then the difference between the normal pressure on a rectangular
element in the lower plane and that on the upper plane is the
weight of the element and the difference between the shears on the vertical faces of
that element. The weights being known, the principal
stresses may be determined. These researches led to a wide
discussion of the sufficiency of the law of uniformly varying
stress when applied to horizontal joints as a test of the stability
of dams. Professor Karl Pearson showed that the results are
dependent upon the assumption that the distribution of the vertical
stresses on the base of the structure also followed the law of
uniformly varying stress. In view of the irregular forms and the
uncertainties of the nature of the materials at the foundation, the
law of uniformly varying stress was not applicable to the base of
the dam. He stated that it wa g
practically impossible to determine the stresses by purely
mathematical means. The late Sir Benjamin Baker, F.R.S.,
suggested that the stresses might be measured by experiments with
elastic models, and among others, experiments were carried out by
Messrs Wilson and Gore a with indiarubber models of plane
sections of dams (including the foundations) who applied
forces to represent the gravity and water pressures in such a
manner that the virtual density of the rubber was increased many
times without interfering with the proper ratio between gravity and
water pressure, and by this means the strains produced were of
sufficient magnitude to be easily measured.

The more important of their results are shown graphically in
figs. 15 and 16, and prove that the law of uniformly varying stress
is generally applicable to the upper two-thirds of a dam, but that
at parts in or near the foundations that law is departed from in a
way which will be best understood from the diagrams.

Fig. 15 shows a section of the model dam. The maximum principal
stresses are represented by the directions and thicknesses of the
two systems of intersecting lines mutually at right angles.

Tensile stresses (indicated by broken lines on the diagram) are
shown at the upstream toe notwithstanding that the line of
resistance is well within the middle third of the section. It is
important to notice that the maximum value of the tension at the
toe lies in a direction approximately at 45° to the vertical, but
at points lower down in the foundation this tension, while less in
magnitude, becomes much more horizontal. This feature indicates
that in the event of a crack occurring at the upstream toe, its
extension would tend to turn downwards and follow a direction
nearly parallel with the maximum pressure lines, in which direction
it would not materially affect the stability of the structure.

As a matter of fact, the foundations of most dams are carried
down in vertical trenches, the lower part only being in sound
materials so that actual separation almost corresponding with the
hypothetical On Some Disregarded Points in the Stability of
Masonry Dams, Drapers' Company Research Memoir (London,
1904).

Engineering (May 12th, 1905).

Proceedings of the Institution of Civil Engineers, vol.
172, p. 107.

crack is allowed in the first instance with no harmful effects.
Similar experiments upon models with rounded toes but otherwise of
the same form showed a considerable reduction in the magnitude of
the tensile stresses.

On examining the diagram it will be observed that the maximum
compressive stresses are parallel to and near to the down stream
face of the section, which values are approximately equal to the
maximum value of the vertical stress determined by the law of
uniformly varying stress divided by the cosine squared of the angle
between the vertical and the resultant.

The distributions of stress on the base line of the model for "
reservoir empty " and " reservoir full " are shown in fig. 16 by
ellipses of stress and by diagrams of stress on vertical and
horizontal sections.

Arrow heads at the ends of an axis of an ellipse indicate tension as distinct from
compression, and the semi-axes in magnitude and direction represent
the principal stresses.

The two systems of lines mutually at right angles show the
directions of the maximum and minimum stresses respectively. Such
stresses. are termed principal stresses. Tension is indicated by
broken lines and compression by full lines.

The shearing stresses are zero along the lines of principal
stress and reach a maximum on lines at 45° thereto. The magnitudes
of the maximum shearing stresses are indicated by the algebraic
differences of the thicknesses of the lines of principal
stress.

Line ab is in such a position that the stresses. along
and above it are not materially affected by the more irregular
stresses below that line produced by the sudden change in section
at the base of the dam. The vertical distance above the line
ab of any point in the dotted line dc is
proportional to the vertical component of the compressive stress on
the line ale assumed to
vary uniformly from face to face, and similarly the vertical
distance of any point in the3-dot-and-dash line ae above
the line ab is proportional to the vertical component of
the stress determined experimentally. The vertical component
diagrams abed and abea are drawn to a larger
scale than t` the lines indicating the principal. stresses.

It is obvious that experiments of the kind referred to cannot
take into account all the conditions of the problem met with in
actual practice, such as the effect of the rock at the sides of the
valley and variations of temperature, &c., but deviations in
practice from the conditions which mathematical analyses or
experiments assume are nearly always present. Such analyses and
experiments are not on that account the less important and
useful.

Missing imageWatersupply-14.jpg

Missing imageWatersupply-15.jpg

So far we have only considered water-pressure against the
reservoir side of the dam; but it sometimes happens that the water
and earth pressure against the outer face is considerable enough to
modify the lower part of the section. In dams of moderate height
above ground and considerable depth below ground there is,
moreover, no reason why advantage should not be taken of the earth
resistance due either to the downstream face of the trench against
which the foundations are built, or to the materials excavated and
properly embanked against that face above the ground level or to
both. We do not always know the least resistance which it is safe
to give to a retaining wall subject to the pressure of earth, or
conversely, the maximum resistance to side-thrust which natural or
embanked earth will afford, because we wisely neglect the important
but very variable element of adhesion between the particles. It is
notorious among engineers that retaining walls designed in
accordance with the well-known theory of conjugate pressures in
earth are unnecessarily strong, and this arises mainly from the
assumption that the earth is merely a loose granular mass without
any such adhesion. As a result of this theory, in the case of a
retaining wall supporting a vertical face of earth beneath an
extended horizontal plane level with the top of the wall, we get p
_ wx 2 1 - sin ii 2 I +sin P'
[[Reservoir Empty Reservoir Full Ellipses Of
Vertical Pressures On Horizontal Joints]].

iiai/iiiiiii?iiiiii where is the horizontal pressure of the
earth against the wall exerted at one-third its height, the weight
of unit volume of the material, the height of the wall, and the
angle of repose of the material. That the pressure so given exceeds
the maximum possible pressure we do not doubt; and, conversely, if
we_put wx 2 I +sin 2 .I - sin ¢' we may have equal confidence that
will be less than the maximum pressure which, if exerted by the
wall against the earth, will be borne without disturbance. But like
every pure theory the principles of conjugate pressures in earth
may lead to danger if not applied with due consideration for the
angle of repose of the material, the modifications brought about by
the limited width of artificial embankments, the possible
contraction away from the masonry, of clayey materials during dry
weather for some feet in depth and the tendency of surface waters
to produce scour between the wall and the embankment. Both the
Neuadd and the Fisher Tarn dams
are largely dependent upon the support of earthen embankments with
much economy and with perfectly satisfactory results. In the
construction of the Vyrnwy
masonry dam Portlandcement concrete was used in the
joints. When more than six months old, 9 in. cubes of this material
never failed under compression below Iii tons per sq. ft. with an
average of 167 tons; and the mean resistance of all the blocks
tested between two and three years after moulding exceeded 215 tons
per sq. ft., while blocks cut from the concrete of the dam gave
from 181 to 329 tons per sq. ft. It has been shown that the best
hydraulic lime, or volcanic puzzuolana and
lime, if properly ground while slaking, and otherwise treated in
the best-known manner, as well as some of the so-called natural
(calcareous) cements, will yield results certainly not inferior to
those obtained from Portland cement. The only objection that can in
any case be urged against most of the natural products is that a
longer time is required for induration; but in the case of masonry
dams sufficient time necessarily passes before any load, beyond
that of the very gradually increasing masonry, is brought upon the
structure. The result of using properly treated natural limes is
not to be judged from the careless manner in which such limes have
often been used in the past. Any stone of which it is desirable to
build a masonry dam would certainly pcssess. an average strength at
least as great as the above figures for concrete; the clay slate of
the Lower. Silurian formation, used' in the case of the Vyrnwy dam,
had an ultimate crushing strength of from 700 to 1000 tons per sq.
ft. If, therefore, with such materials the work is well done, and
is not subsequently liableto be wasted or disintegrated by
expansion or contraction or other actions which in the process of
time affect all exposed surfaces, it is clear that 15 to 20 tons
per sq. ft. crust be a perfectly safe load. There are many
structures at present in existence bearing considerably greater
loads than this, and the graniteashlar masonry of at least one, the Bear Valley
dam in California, is
subject to compressive stresses, reaching, when the reservoir is
full, at least 40 to 50 tons per sq. ft., while certain brickwork
linings in mining shafts are subject to very high circumferential
stresses, due to known water-pressures. In one case which has been
investigated this circumferential pressure exceeds 26 tons per sq.
ft., and the brickwork, which is 18 in. thick. and 20 ft. internal
diameter, is perfectly sound and water-tight. In portions of the
structure liable to important changes of pressure from the rise,
and fall of the water and subject to the additional stresses which
expansion and contraction by changes of temperature and of moisture
induce, and in view of the great difficulty of securing that the
average modulus of elasticity in all parts of the structure
shall be approximately the same, it is probably desirable to limit
the calculated load upon any external work, even of the best kind,
to 15 or 20 tons per sq. ft. It is clear that the material upon
which any high masonry dam is founded must also have a large factor
of safety against crushing under the greatest load that the dam can
impose upon it, and this consideration unfits any site for the
construction of a masonry dam where sound rock, or at least a
material equal in strength to the strongest shale, cannot be had;
even in the case of such a material as shale the foundation must be
well below the ground.

The actual construction of successful masonry dams has varied
from the roughest rubble masonry to ashlar work. It. is probable,
however, that, all things considered, Materialsrandom rubble in which the
flattest side of each block of stone is dressed to a fairly uniform
surface, so that it may be bedded as it were in a tray of mortar, secures the nearest approach to uniform
elasticity. Such stones may be of any size subject to each of them
covering only a small proportion of the width of the structure (in
the Vyrnwy dam they reached 8 or 10 tons each), and the spaces
between them, where large enough, must be similarly built in with
smaller, but always the largest possible, stones; spaces too small
for this treatment must be filled and rammed with concrete. All
stones must be beaten down into their beds until the mortar
squeezes up into the joints around them. The faces of the work may
be of squared masonry, thoroughly tied into the hearting; but, in
view of the expansion and contraction mentioned below, it is better
that the face masonry should not be coursed. Generally speaking, in
the excavations for the foundations springs are met with; these may
be only sufficient to indicate a continuous dampness at certain
beds or joints of the rock, but all such places should be connected
by relief drains carried to visible points at the back of the dam.
It should be impossible, in short, for any part of the rock beneath
the dam to become charged with water under pressure, either
directly from the water in the reservoir or from higher places in
the mountain
sides. For similar reasons care must be taken to ensure that the
structure of the water face of the dam shall be the least permeable
of any part. In the best examples this has been secured by bedding
the stones near to the water face in somewhat finer mortar than the
rest, and sometimes also by placing pads to fill the joints for
several inches from the water face, so that the mortar was kept
away from the face and was well held up to its work. On the removal
of the pads, or the cutting out of the face of the mortar where
pads were not used, the vacant joint was gradually filled with
almost dry mortar, a hammer
and caulking tool being used to consolidate it. By these means
practical impermeability was obtained. If the pores of the water
face are thus rendered extremely fine, the surface water, carrying
more or less fine detritus and organic matter, will soon close them
entirely and assist in making that face the least permeable portion
of the structure.

But no care in construction can prevent the compression of the
mass as the superincumbent weight comes upon it. Any given yard of
height measured during construction, or at any time after
construction, will be less than a yard when additional weight has
been placed upon it; hence the ends of such dams placed against
rock surfaces must move with respect to those surfaces when the
superincumbent load comes upon them. This action is obviously much
reduced where the rock sides of the valley rise slowly; but in
cases where the rock is very steep, the safest course is to face
the facts, and not to depend for water-tightness upon the cementing
of the masonry to the rock, but rather to provide a vertical key, or dowel joint, of some material
like asphalt, which will always remain water-tight. So far as the
writer has been able to observe or ascertain, there are very few
masonry dams in Europe or America which have not been cracked
transversely in their higher parts. They generally leak a little
near the junction with the rock, and at some other joints in
intermediate positions. In the case of the Neuadd dam this
difficulty was met by deliberately omitting the mortar in
transverse joints at regular intervals near the top of the dam,
except just at their faces, where it of course cracks harmlessly,
and by filling the rest with asphalt. Serious movement from
expansion and contraction does not usually extend to levels which
are kept moderately damp, or to
the greater mass of the dam, many feet below high-water level.

The first masonry dam of importance constructed in Great Britain
was that upon the river Vyrnwy, a tributary of the Severn, in
connexion with the Liverpool water-supply (Plate I.). Its height,
subject to water-pressure, is about 134 ft., and a carriage-way is carried on
arches at an elevation of about 18 ft. higher. As this dam is about
1180 ft. in length from rock to rock, it receives practic ally no
support from the sides of the valley. Its construction drew much
attention to the subject of masonry dams in England - where the
earthwork dam, with a wall of puddled clay, had hitherto been
almost universal - and since its completion nine more masonry dams
of smaller size have been completed. In connexion with the Elan and
Claerwen works, in Mid-Wales, for the supply of Birmingham, six masonry
dams were projected, three of which are completed, including the
Caban Goch dam, 590 ft. long at
the water level, and subject to a water-pressure of 152 ft. above
the rock foundations and of 122 ft. above the river bed, and the
Craig-yr-allt Goch dam, subject to a head of 133 ft. The latter dam
is curved in plan, the radius being 740 ft. and the chord of the
arc 515 ft. In the Derwent
Valley scheme, in connexion with the water supplies of Derby, Leicester, Nottingham and Sheffield, six more masonry dams have
received parliamentary sanction. Of these the highest is the
Hagglee, on the Ashop, a tributary of the Derwent, which will
impound water to about 136 ft. above the river bed, the length from
rock to rock being 980 ft. Two of these dams are now in course of
construction, one of which, the Howden, will be 1080 ft. in length
and will impound water to a depth of 114 ft. above the river bed.
In 1892 the excavation was begun for the foundations of a masonry
dam across the Croton river, in connexion with the supply of New York. The length of this
dam from rock to rock at the overflow level is about 1500 ft. The
water face, over the maximum depth at which that face cuts the rock
foundations, is subject to a water-pressure of about 260 ft., while
the height of the dam above the river bed is 163 ft. The section,
shown in fig. 17, has been well considered. The hearting is of
rubble masonry, and the faces are coursed ashlar.

FIG. 17. - Section of Croton Dam.

So-called " natural cement " has been used, except during frosty
weather, when Portland cement was substituted on account of its
more rapid setting. An important feature in connexion with this dam
is the nature of the foundation upon which it stands. Part of the
rock is schist, but the greater portion limestone, similar in
physical qualities to the Carboniferous limestone of Great
Britain. The lowest part of the surface of this rock was reached
after excavating through alluvial deposits to a depth of about 70
ft., but owing to its fissured and cavernous nature it became
necessary to excavate to much greater depths, reaching in places
more than 120 ft. below the original bottom of the valley. Great
pains appear to have been taken to ascertain that the cavernous
portions of the rock had been cut out and built up before the
building was begun.

PLATE.

The Furens dam, already referred to as the earliest type of a
scientifically designed structure of the kind, is subject to a
pressure of about 166 ft. of water; the valley it crosses is only
about 300 ft. wide at the water level, and the dam is curved in
plan to a radius of 828 ft. Much discussion has taken place as to
the utility of such curvature. The recent investigations already
referred to indicate the desirability of curving dams in plan in
order to reduce the possibility of tension and infiltration of
water at the upstream face. In narrow rock gorges extremely
interesting and complex problems relating to the combined action of
horizontal and vertical stresses arise, and in some such cases it
is evident that much may be done by means of horizontal curvature
to reduce the quantity of masonry without reduction of strength.
The Bear Valley dam, California, is the most THE Vyrnwy Valley,
Montgomeryshire, June 1888.

From Photographs by J, Maclardy. Lake Vyrnwy, December
1889.

XXVIII. 402.

daring example in existence of the employment of the arch
principle. Its height from the rock bed is 64 ft., and it is
subject during floods to a head of water not much less. The length
of the chord of the arc across the valley is about 250 ft. and the
radius 335 ft. The dam was begun in 1883, with a base 20 ft. thick,
narrowing to 13 ft. at a height of 16 ft. The cost of this
thickness being regarded as too great, it was abruptly reduced to 8
ft. 6 in., and for the remaining 48 ft. it was tapered up to a
final width of about 3 ft. The masonry is described by Mr Schuyler
as " a rough uncut granite ashlar, with a hearting of rough rubble
all laid in cement mortar and gravel." This dam has been in
satisfactory use since 1885, and the slight filtration through the
masonry which occurred at first is said to have almost entirely
ceased.

In New South
Wales thirteen thin concrete dams, dependent upon horizontal
curvature for their resistance to water pressure, have been
constructed in narrow gorges at comparatively small cost to impound
water for the use of villages. The depth of water varies from 18
ft. to 76 ft. and five of them have cracked vertically, owing
apparently to the impossibility of the base of the dam partaking of
the changes of curvature induced by changes of temperature and of
moisture in the upper parts. It is stated, however, that these
cracks close up and become practically water-tight as the water
rises.

Something has been said of the failures of earthen dams. Many
masonry dams have also failed, but, speaking generally, we know
less of the causes which have led to such failures. The
Failures. examination of one case, however, namely, the
bursting in 1895 of the Bouzey dam, near Epinal, in France, by
which many lives were lost, has brought out several points of great
interest. It is probably the only instance in which a masonry dam
has slipped upon its foundations, and also the only case in which a
masonry dam has actually overturned, while curiously enough there
is every probability that the two circumstances had no connexion
with each other. A short time after the occurrence of the catastrophe the dam was
visited by Dr W. C. Unwin, F.R.S., and the writer, and a very
careful examination of the work was made by them. Some of the
blocks of rubble masonry carried down the stream weighed several
hundred tons. The original section of the dam is shown by the
continuous thick line in fig. 18, from which it appears that the
work was subject to a pressure of only about 65 ft. of water. In
the year 1884 a length FIG. 18. - Section of Bouzey Dam.

of 450 ft. of the dam, out of a total length of 1706 ft.,
slipped upon its foundation of soft sandstone, and became slightly
curved in plan as shown at a, b, fig. 19, the maximum
movement from the original straight line being about I ft. Further
sliding on the base was prevented by the construction of the
cross-lined portions in the section (fig. 18). These precautions
were perfectly effective in securing the safety of the dam up to
the height to which the counterfort was carried. As a consequence
of this horizontal bending of the dam the vertical cracks shown in
fig. 19 appeared and were repaired. Eleven 1 See PrOC. Inst.
C.E. vol. cxxvi. pp. 91-95.

years after this, and about fifteen years after the dam was
first brought into use, it overturned on its outer edge, at about
the level indicated by the dotted line just above the counterfort;
and there is no good reason to attribute to the movement of 1884,
or to the vertical cracks it caused, any influence in the
overturning of 1895. Some of the worst cracks were, indeed,
entirely beyond the portion overturned, which consisted of the mass
570 ft. long by 37 ft. in depth, and weighing about 20,000 tons,
shown in elevation in fig. 19. The line of pressures as generally
given for this dam with the reservoir full, on the hypothesis that the
density of the masonry was a little over 2, is shown by long and
short dots in fig. 18. Materials actu ally collected from the dam
indicate that the mean density did not exceed I. 85 when dry and
2.07 when saturated, which would bring the line of pressures even
closer to the outer face at the top of the counterfort. In any
event it must have approached well within 31 ft. of the outer face,
and was more nearly five-sixths than two-thirds of the width of the
dam distant from the water face; there must, therefore, have been
considerable vertical tension at the water face, variously computed
according to the density assumed at from 14 to I q ton per square
foot. This, if the dam had been thoroughly well constructed, either
with hydraulic lime or Portland cement mortar, would have been
easily borne. The materials, however, were poor, and it is probable
that rupture by tension in a roughly horizontal plane took place.
Directly this occurred, the front part of the wall was subject to
an additional overturning pressure of about 35 ft. of water acting
upwards, equivalent to about a ton per square foot, which would
certainly, if it occurred throughout any considerable length of the
dam, have immediately overturned it. But, as a matter of fact, the
dam actually stood for about fifteen years. Of this circumstance
there are two possible explanations. It is known that more or less
leakage took place through the dam, and to moderate this the water
face was from time to time coated and repaired with cement. Any
cracks were thus, no doubt, temporarily closed; and as the
structure of the rest of the dam was porous, no opportunity was
given for the percolating water to accumulate in the horizontal
fissures to anything like the head in the reservoir. But in
reservoir work such coatings are not to be trusted, and a single
horizontal crack might admit sufficient water to cause an uplift.
Then, again, it must be remembered that although the full
consequences of the facts described might arise in a section of the
dam I ft. thick (if that section were entirely isolated), they
could not arise throughout the length unless the adjoining sections
were subject to like conditions. Any horizontal fissure in a weak
place would, in the nature of things, strike somewhere a stronger
place, and the final failure would be deferred. Time would then
become an element. By reason of the constantly changing
temperatures and the frequent filling and emptying of the
reservoir, expansion and contraction, which are always at work
tending to produce relative movements wherever one portion of a
structure is weaker than another, must have assisted the
water-pressure in the extension of the horizontal cracks, which,
growing slowly during the fifteen years, provided at last the area
required to enable the intrusive water to overbalance the little
remaining stability of the dam.

Reservoirs From very ancient times in India, Ceylon and elsewhere, reservoirs of great area,
but generally of small depth, have been built and used for the
purposes of irrigation; and in modern times, especially in India
and America, comparatively shallow reservoirs have been constructed
of much greater area, and in some cases of greater capacity, than
any in the United Kingdom.

Missing imageWatersupply-17.jpg

Missing imageWatersupply-18.jpg

ELE VATiON ?--- -----570 0' - ?

Missing imageWatersupply-19.jpg

Water Face. FIG. 19. - Elevation and Plan of Bouzey
Dam.

Yet the hilly parts of the last-named country are rich in
magnificent sites at sufficient altitudes for the supply of any
parts by gravitation, and capable, if properly laid out, of
affording a volume of water, throughout the driest seasons, far in
excess of the probable demand for a long future. Many of the great
towns had already secured such sites within moderate distances, and
had constructed reservoirs of considerable size, when, in 1879,
1880 and 1892 respectively, Manchester, Liverpool and Birmingham
obtained statutory powers to draw water from relatively great
distances, viz. from Thirlmere in Cumberland, in the case of Manchester; from
the river Vyrnwy, Montgomeryshire, a tributary of the
Severn, in the case of Liverpool; and from the rivers Elan and
Claerwen in Radnorshire, tributaries of the Wye, in the case of Birmingham. Lake
Vyrnwy, completed in 1889, includes a reservoir which is still by
far the largest in Europe.

This reservoir is situated in a true Glacial lake-basin, and
having therefore all the appearance of a natural lake, is commonly
known as Lake Vyrnwy. It is 825 ft. above the sea, has an
Lake area of 1121 acres, an available capacity exceeding
12,000 Vyrnwy. million gallons, and a length of nearly 5
m. Its position in North Wales
is shown in black in fig. 20, and the two views on Plate I. show
respectively the portion of the valley visible from the dam before
impounding began, and the same portion as a lake on the completion
of the work. Before the valves
in the dam were closed, the village of Llanwddyn, the parish
church, and many farmsteads were demolished. The church was rebuilt
outside the watershed, and the remains from the old churchyard were removed
to a new cemetery
adjoining it. The fact that this valley is a post-Glacial
lake-basin was attested by the borings and excavations made for the
foundations of the dam. The trench in which the masonry was founded
covered an area 120 ft. wide at the bottom, and extending for 1172
ft. across the valley. Its site had been determined by about 190
borings, probings and shafts, which, following upon the indications
afforded by the rocks above ground, proved that the rock bed
crossing the valley was higher at this point than elsewhere. Here
then, buried in alluvium
at a depth of 50 to 60 ft. from the surface, was found the rock bar of the post-Glacial lake; at points
farther up the valley, borings nearly too ft. deep had failed to
reach the rock. The Glacial striae, and the dislocated rocks -
moved a few inches or feet from their places, and others, at
greater distances, turned over, and beginning to assume the
sub-angular form of Glacial boulders - were found precisely as the
glacier, receding from the bar, and giving place to
the ancient lake, had left them, covered and preserved by sand and
gravel washed from the terminal morain. Later came the alluvial
silting-up. Slowly, but surely, the deltas of the tributary streams
advanced into the lake, floods deposited their burdens of detritus
in the deeper places, the lake shallowed and shrank and in its turn
yielded to the winding river of an alluvial strath, covered with peat, reeds and alders, and still
liable to floods. It is interesting to record that during the
construction of the works the implements of Neolithic man were found, near the margin of
the modern lake, below the peat, and above the alluvial clay on
which it rested. Several of the reservoir sites in Wales, shown by
shaded lines in fig. 20, are in all probability similar
post-Glacial lake-basins, and in the course of time some of them
may contain still greater reservoirs. They are provided with
well-proportioned watersheds and rainfall, and being nearly all
more than 500 ft. above the sea, may be made available for the
supply of pure water by gravitation to any part of England. In 1892
the Corporation of Birmingham obtained powers for the construction
of six reservoirs on the rivers Elan and Claerwen, also shown in
fig. 20, but the sites of these reservoirs are long narrow valleys,
not lake-basins. The three reservoirs on the Elan were completed in
1904. Their joint capacity is 11,320 million gallons, and this will
be increased to about 18,000 millions when the remaining three are
built.

Of natural lakes in Great Britain raised above their ordinary
levels that the upper portions may be utilized as reservoirs, Loch Katrine
supplying Glasgow is well
known. Whitehaven is
similarly supplied from Ennerdale, and in the year 1894 Thirlmere
in Cumberland was brought into use, as already mentioned, for the
supply of Manchester. The corporation have statutory power to raise
the lake 50 ft., at which level it will have an available capacity
of about 8000 million gallons; to secure this a masonry dam has
been constructed, though the lake is at present worked at a lower
level.

It is obvious that the water of a reservoir must never be
allowed to rise above a certain prescribed height at which the
works will be perfectly safe. In all reservoirs impounding the
natural flow of a stream, this involves the use of an overflow.
Where the dam is of masonry it may be used as a weir; but where
earthwork is employed, the overflow, commonly known in such a case
as the " bye-wash," should be an entirely independent work,
consisting of a low weir of sufficient length to prevent an unsafe
rise of the water level, and of a narrow channel capable of easily
carrying away any water that passes over the weir. The absence of
one or bath ,of these conditions
has led to the failure of many dams.

Reservoirs unsafe from this cause still exist in the United
Kingdom. Where the contributory drainage area exceeds 5000 acres,
the discharge, even allowing for so-called " cloud-bursts," rarely
or never exceeds the rate of about 300 cub. ft. per second per woo
acres, or 1500 times the minimum dry weather flow, taken as
one-fifth of a cubic foot; and if we provide against such an
occasional discharge, with a possible maximum of 400 cub. ft. at
much more distant intervals, a proper factor of safety will be
allowed. But when a reservoir is placed upon a smaller area the
conditions are materially changed. The rainfall which produces, as
the average of all the tributaries in the larger area, 300 cub. ft.
per second per woo acres, is made up of groups of rainfall of very
varying intensity, falling upon different portions of that area, so
that upon any section of it the intensity of discharge may be much
greater.

The height to which the water is permitted to rise above the sill of the overflow depends upon
the height of the embankment above that level (in the United
Kingdom commonly 6 or 7 ft.), and this again should be governed by
the height of possible waves. In open places that height is seldom
more than about one and a half times the square root of the " fetch
" or greatest distance in nautical miles from which the wave has travelled to the point in
question; but in narrow reaches or lakes it is relatively higher.
In lengths not exceeding about 2 m., twice this height may be
reached, giving for a 2-mile " fetch " about 32 ft., or 14 ft.
above the mean level. Above this again, the height of the wave
should be allowed for " wash," making the embankment in such a case
not less than 54 ft. above the highest water-level. If, then, we
determine that the depth of overflow shall not exceed 1 ft., we
arrive at 64 ft. as sufficient for the height of the embankment
above the sill of the overflow. Obviously we may shorten the sill
at the cost of extra height of embankment, but it is rarely wise to
do so.

The overflow sill or weir should be a masonry structure of
rounded vertical section raised a foot or more above the
waste-water course, in which case for a depth of t a ft. it will
discharge, over every foot of length, about 6 cub. ft. per second.
Thus, if the drainage area exceeds 5000 acres, and we provide for
the passage of 300 cub. ft. per second per woo acres, such a weir
will be 50 ft. long for every 1000 acres. But, as smaller areas are
approached, the excessive local rainfalls of short duration must be
provided for, and beyond these there are extraordinarily heavy
discharges generally over and gone before any exact records can be
made; hence we know very little of them beyond the bare fact that
from woo acres the discharge may rise to two or three times 300
cub. ft. per second per woo acres. In the writer's experience at
least one case has occurred where, from a mountain area of 1300
acres, the rate per woo was for a short time certainly not less
than woo cub. ft. per second. Nothing but long observation and
experience can help the hydraulic engineer to judge of the
configuration of the ground favourable to such phenomena. It is
only necessary, however, to provide for these exceptional
discharges during very short periods, so that the rise in the
water-level of the reservoir may be taken into consideration; but
subject to this, provision must be made at the bye-wash for
preventing such a flood, however rare, from filling the reservoir
to a dangerous height.

From the overflow sill the bye-wash channel may be gradually
narrowed as the crest of the embankment is
passed, the water being prevented from attaining undue velocity by
steps of heavy masonry, or, where the gradient is not very steep,
by irregularly set masonry.

Purification
When surface waters began to be used for potable purposes, some
mode of arresting suspended matter, whether living or dead, became
necessary. In many cases gauze strainers were at first employed,
and, as an improvement upon or addition to these, the water was
caused filtratio n. to pass through a bed of gravel or
sand, which, like the gauze, was regarded merely as a strainer. As
such strainers were further improved, by sorting the sand and
gravel, and using the fine sand only at the surface, better
clarification of the water was obtained; but chemical analysis
indicated, or was at the time thought to indicate, that that
improvement was practically confined to clarification, as the
dissolved impurities in the water were certainly very little
changed. Hence such filter
beds, as they were even then called, were regarded as a luxury
rather than as a necessity, and it was never suspected that,
notwithstanding the absence of chemical improvement in the water,
changes did take place of a most important kind. Following upon Dr
Koch's discovery of a method of isolating bacteria, and of making
approximate determinations of their number in any volume of water,
a most remarkable diminution in the number of microbes contained in
sand-filtered water was observed; and it is now well known that
when a properly. constructed sand-filter bed is in its best
condition, and is worked in the best-known manner, nearly the whole
of the microbes existing in the crude water will be arrested. The
sand, which is nominally the filter, has interstices about thirty
times as wide as the largest dimensions of the larger microbes; and
the reason why these, and, still more, why organisms which were
individually invisible under any magnifying power, and could only
be detected as colonies, were arrested, was not understood. In
process of time it became clear, however, that the worse the
condition of a filter bed, in the then general acceptation of the
term, the better it was as a microbe filter; that is to say, it was
not until a fine film of mud and microbes had formed upon the
surface of the sand that the best results were obtained.

Even yet medical science has not determined the effect upon the
human system of water highly charged with bacteria which are not
known to be individually pathogenic. In the case of the bacilli of
typhoid and cholera, we know
the direct effect; but apart altogether from the presence of such
specific poisons, polluted water is undoubtedly injurious. Where,
therefore, there is animal pollution of any kind, more especially
where there is human pollution, generally indicated by the presence
of bacillus coli communis, purification is of supreme
importance, and no process has yet been devised which, except at
extravagant cost, supersedes for public supplies that of
properly-conducted sand filtration. Yet it cannot be too constantly
urged that such filtration depends for its comparative perfection
upon the surface film; that this surface film is not present when
the filter is new, or when its materials have been recently washed;
that it may be, and very often is, punctured by the actual working
of the filters, or for the purpose of increasing their discharge;
and that at the best it must be regarded as an exceedingly thin
line of defence, not to be depended upon as a safeguard against
highly polluted waters, if a purer source of supply can possibly be
found. Such filters are not, and in the nature of things cannot be,
worked with the precision and continuity of a laboratory
experiment.

In fig. 21 a section is shown of an efficient sand-filter bed.
The thickness of sand is 3 ft. 6 in. In the older filters it was
usual to support this sand upon small gravel resting upon larger
gravel, and so on until the material was sufficiently open to pass
the water laterally to underdrains. But a much shallower and
certainly not less efficient filter can be constructed by making
the under-drains cover the whole bottom. In fig. 21 the sand rests
on small gravel of such degree of coarseness that the whole of the
grains would be retained on a sieve of a-in.,mesh and rejected by a sieve of
z-in. mesh in the clear, supported upon a 3-in. thickness of bricks
laid close together, and consti BRIeKS. tuting the roof of the
under '" - a,:;z;,=?.?i,???
drains, which are formed by Concrete. other bricks laid on thin
asphalt, upon a concrete FIG. 2 I. - Section of Sand-Filter Bed.
floor. In this arrangement the whole of the materials may be
readily removed for cleansing. In the best filters an automatic
arrangement for the measurement of the supply to each separate
filter, and for the regulation of the quantity within certain
limits, is adopted, and the resistance at outflow is so arranged
that not more than a certain head of pressure, about 22 ft., can
under any circumstances come upon the surface film, while a depth
of several feet of water is maintained over the sand. It is
essential that during the working of the filter the water should be
so supplied that it will not disturb the surface of the sand. When
a filter has been emptied, and is being re-charged, the water
should be introduced from a neighbouring filter, and should pass
upwards in the filter to be charged, until the surface of the sand
has been covered. The unfiltered water may then be allowed to flow
quietly and to fill the space above the sand to a depth of 2 or 3
ft. It would appear to be impossible with any water that requires
filtration to secure that the first filtrate shall be satisfactory
if filtration begins immediately after 'a filter is charged; and if
the highest results are to be obtained, either the unfiltered water
must be permitted to pass extremely slowly over the surface of the
sand without passing through it, or to stand upon the sand until
the surface film has formed. With waters giving little or no
sediment, which are often the most dangerous, some change, as by
the first method, is necessary. It has been proposed, on the other
hand, to allow the filter to act slowly until the surface film is
formed, and to discard the first effluent. This course can scarcely
fail to introduce into the sand many bacteria, which may be washed
through when the full working of the filters is begun; and it
should not, therefore, be adopted when the source of the supply is
known to be subject to human pollution. The time for the formation
of an efficient surface films varies, according to the quality of
the raw water, from a few hours to a few days. Judging from the
best observations that have been made on a large scale, the highest
rate of efficient filtration when the surface film is in good
condition is about 4 in. downwards per hour of the water contained
above the sand, equivalent to about 50 gallons per day from each
square foot of sand. When the surface film has once been formed,
and the filter has begun its work, it should continue without
interruption until the resistance of that film becomes too great to
permit of the necessary quantity of water being passed. That period
will vary, according to the condition of the water, from eight or
ten days to four weeks. The surface film, together with half an
inch to an inch of sand, is then carefully scraped off and stored
for subsequent washing and use. This process may be repeated many
times until the thickness of the fine sand is reduced to about 18
in., when the filter bed should be restored to its full
thickness.

A lately discovered effect of sand filtration is a matter of
great importance in connexion with the subject of aqueducts. A
brown slimy sediment, having the appearance of coffee grounds when placed in clear water, has
been long observed in pipes conveying surface waters from mountain
moorlands. The deposit grows
on the sides of the pipes and accumulates at the bottom, and causes
most serious obstruction to the flow of water. The chemists and
bacteriologists do not appear to have finally determined the true
nature and origin of this growth, but it is found in the impounded
waters, and passes into the pipes, where it rapidly increases. It
is checked even by fine copper wire-gauze strainers, and where the
water passes through sand-filter beds in the course of an aqueduct, the growth, though
very great between the reservoir and the filter beds, is almost
absent between the filter beds and the town. Even the growth of the
well-known nodular incrustations in iron pipes is much reduced by
sand filtration. From these facts it is clear that, other things
being the same, the best position for the strainers and filter beds
is as close as possible to the reservoir.

Some surface waters dissolve lead when bright, but cease to do
so when the lead becomes tarnished. More rarely the action is
continuous, and the water after being passed through lead cisterns
and pipes produces lead poisoning - so called " plumbism."
The liability to this appears to be entirely removed by efficient
sand filtration.

Sand filtration, even when working in the best possible manner,
falls short of the perfection necessary to prevent the passage of
bacteria which may multiply after the filter is passed. Small,
however, as the micro-organisms are, they are larger than the
capillary passages in some materials through which water under
pressure may be caused to percolate. It is therefore natural that
attempts should have been made to construct filters which, while
permitting the slow percolation of water, should preclude the
passage of bacteria or their spores. In the laboratory of Pasteur
probably the first filter which successfully accomplished this
object was produced. In this apparatus, known as the
Pasteur-Chamberland filter, the filtering medium is biscuitporcelain. It was followed by the Berkefield
filter, constructed of baked infusorial earth. Both these filters
arrest the organisms by purely
mechanical action, and if the joints are water-tight and they
receive proper attention and frequent sterilization, they both give
satisfactory results on a small scale for domestic purposes. The
cost, however - to say nothing of the uncertainty - where large
volumes of water are concerned, much exceeds the cost of obtaining
initially safe water. Moreover, if a natural water is so liable to
pathogenic pollution as to demand filtration of this kind, it ought
at once to be discarded for an initially pure supply; not
necessarily pure in an apparent or even in a chemical sense, for
water may be visibly coloured, or may contain considerable
proportions both of organic and inorganic impurity, and yet be
tasteless and free from pathogenic pollution.

There are several materials now in use possessing remarkable
power to decolourize clarify, chemically purify and oxidize water;
but they are too costly for use in connexion with public water
supplies unless a rate of filtration is adopted quite inconsistent
with the formation of a surface film capable of arresting
micro-organisms. This fact does not render them less useful when
applied to the arts in which they are successfully employed.

Attempts have been made, by adding certain coagulants to the
water to be filtered, to increase the power of sand and other
granular materials to arrest bacteria when passing through them at
much higher velocities than are possible for successful filtration
by means of the surface film upon sand. The effect is to produce
between the sand or other grains a glutinous substance which does
the work performed by the mud and microbes upon the surface of the
sand filter. Elsewhere centrifugal force, acting somewhat after its
manner in the cream separator, has been called in aid.

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waters; but the first cost of the works and the subsequent
removal of the sediment are in some cases a serious matter, and any
approach to the comparatively perfect action of lakes is out of the
question. By the use of such tanks, however, when the condition of
the water demands it, and by passing the effluent water through
sand filters when in good condition, the number of microbes is
found to be reduced by as much as 97 or even 99%. This, when
attained, is undoubtedly a most important reduction in the chance of pathogenic bacteria
passing into the filtered water; but much mere must be done than
has hitherto in most places been done to ensure the constancy of
such a condition before it can be assumed to represent the degree
of safety attained. No public supply should be open to any such
doubt as ought to, or may, deter people from drinking the water
without previous domestic filtration or boiling.

Inter- cast-lead pipes, but they were regarded as
luxuries, mittent supply , not as necessaries, and gave
way to cheaper conduits made, as pump barrels had long been made,
by boring out tree trunks, which are occasionally dug up in a
good state of preservation. This use of tree-trunks as pipes is
still common in the wooded mountain districts of Europe. Within the
19th century, however, cast iron became general in the case of
large towns; but following the precedent inseparable from the use
of weaker conduits, the water was still delivered under very low
pressure, rarely more than sufficient to supply taps or tanks near
the level of the ground, and generally for only a short period out
of each twenty-four hours. On the
introduction of the Waterworks Clauses Act 1847, an impetus was
given to high-pressure supplies, and the same systems of
distributing mains were frequently employed for the purpose; but
with few exceptions the water continued to be supplied
intermittently, and cisterns or tanks were necessary to store it
for use during the periods of intermission. Thus it happened that
pipes and joints intended for a low-pressure supply were subjected,
not only to high pressure, but to the trying ordeal of suddenly varying pressures. As a rule
such pipes were not renewed: the leakage was enormous, and the
difficulty was met by the very inefficient method of reducing the
period of supply still farther. But even in entirely new
distributing systems the network is so extensive, and the number of
joints so great, that the aggregate leakage is always considerable;
the greatest loss being at the so-called " ferrules " connecting
the mains with the house " communication " or " service " pipes, in
the lead pipes, and in the household fittings. But a far greater
evil than mere loss of water and inconvenience soon proved to be
inseparable from intermittent supply. Imagine a hilly town with a
high-pressure water supply, the water issuing at numerous points,
sometimes only in exceedingly small veins, from the pipes into the
sub-soil. In the ordinary course of intermittent supply or for the
purpose of repairs, the
water is cut off at some point in the main above the leakages; but
this does not prevent the continuance of the discharge in the lower
part of the town. In the upper part there is consequently a
tendency to the formation of a vacuum, and some of the impure
sub-soil water near the higher leakages is sucked into the mains,
to be mixed with the supply when next turned on. We are indebted to
the Local Government Board for
having traced to such causes certain epidemics of typhoid, and
there can be no manner of doubt that the evil has been very
general. It is therefore of supreme importance that the pressure
should be constantly maintained, and to that end, in the
best-managed waterworks the supply is not now cut off even for the
purpose of connecting house-service pipes, an apparatus being
employed by which this is done under pressure. Constant pressure
being granted, constant leakage is inevitable, and being constant
it is not surprising that its total amount often exceeds the
aggregate of the much greater, but shorter, draughts of water taken for various household
purposes. There is therefore, even in the best cases, a wide field
for the conservation and utilization of water hitherto entirely
wasted.

Following upon the passing of the Waterworks Clauses Act 1847, a
constant supply was attempted in many towns, with the result in
some cases that, owing to the enormous loss arising from the
prolongation of the period of leakage from [[[Distribution]] a
fraction of an hour to twenty-four hours, it was impossible to
maintain the supply. Accordingly, in some places large sections of
the mains and service pipes were entirely renewed,
Constant and the water consumers were put to great expense
in supply. changing their fittings to new and no doubt
better types, though the old fittings were only in a fraction of
the cases actually causing leakage. But whether or not such
stringent methods were adopted, it was found necessary to organize
a system of house-to-house visitation and constantly recurring
inspection. In Manchester this was combined with a most careful
examination, at a depot of the
Corporation, of all fittings intended to be used. Searching tests
were applied to these fittings, and only those which complied in
every respect with the prescribed regulations were stamped and
permitted to be fixed within the limits of the water supply. But
this did not obviate the necessity for house - to-house inspection,
and although the number of different points at which leakage
occurred was still great, it was always small in relation to the
number of houses which were necessarily entered by the inspector;
moreover, when the best had been done that possibly could be done
to suppress leakage due to domestic fittings, the leakage below
ground in the mains, ferrules and service pipes still remained, and
was often very great. It was clear, therefore, that in its very
nature, house-to-house visitation was both wasteful and
insufficient, and it remained for Liverpool to correct the
difficulty by the application, in 1873, of the " Differentiating
waste water meter," which has since been extensively used for the
same purpose in various countries. One such instrument was placed
below the roadway upon each main supplying a population of
generally between 000 and 2000 persons.

Its action is based upon the following considerations: When
water is passing through a main and supplying nothing but leakage
the flow of that water is necessarily uniform, and any instrument
which graphically represents that flow as a horizontal line conveys
to the mind a full conception of the nature of the flow, and if by
the position of that line between the bottom and the top of a
diagram the quantity of water (in gallons per hour, for example) is
recorded, we have a full statement, not only of the rate of flow,
but of its nature. We know, in short, that the water is not being
usefully employed. In the actual instrument, the paper diagram is
mounted upon a drum caused by
clockwork to revolve uniformly, and is ruled with vertical hour
lines, and horizontal quantity lines representing gallons per hour.
Thus, while nothing but leakage occurs the uniform horizontal line
is continued. If now a tap is opened in any house connected with
the main, the change of flow in the main will be represented by a
vertical change of position of the horizontal line, and when the
tap is turned off the pencil
will resume its original vertical position, but the paper will have
moved like the hands of a clock
over the interval during which the tap was left open. If, on the
other hand, water is suddenly drawn off from a cistern supplied
through a ball-cock, the flow
through the ball-cock will be recorded, and will be represented by
a sudden rise to a maximum, followed by a gradual decrease as the
ball rises and the cistern fills; the result being a curve having
its asymptote in the original horizontal line. Now, all the uses of
water, of whatever kind they may be, produce some such irregular
diagrams as these, which can never be confused with the uniform
horizontal line of leakage, but are always superimposed upon it. It
is this leakage line that the waterworks engineer uses to ascertain
the truth as to the leakage and to assist him in its suppression.
In well-equipped waterworks each house service pipe is controlled
by a stop-cock accessible from the footpath to the officials of the
water authority, and the process of waste detection by this method
depends upon the manipulation of such stop-cocks in conjunction
with the differentiating meter. As an example of one mode of
applying the system, suppose that a night inspector begins work at
11.30 p.m. in a certain district of 2000 persons, the meter of
which records at the time a uniform flow of 2000 gallons an hour,
showing the not uncommon rate of leakage of 24 gallons per head per
day. The inspector proceeds along the footpath from house to house,
and outside each house he closes the stop-cock, recording opposite
the number of each house the exact time of each such operation.
Having arrived at the end of the district he retraces his steps,
reopens the whole of the stop-cocks, removes the meter diagram,
takes it to the night complaint office, and enters in the " night
inspection book " the records he has made. The next morning the
diagram and the " night inspection book " are in the hands of the
day inspector, who compares them. He finds, for example, from the
diagram that the initial leakage of 2000 gallons an hour has in the
course of a 41 hours' night inspection fallen to 400 gallons an
hour, and that the 1600 gallons an hour is accounted for by Distribution The
earliest water supplies in Great Britain were generally distributed
at low pressure by wooden pipes or stone or brick conduits. For
special purposes the Romans
introduced Detection of waste. fifteen distinct drops of
different amounts and at different times. Each of these drops is
located by the time and place records in the book and the time
records on the diagram as belonging to a particular service pipe;
so that out of possibly 300 premises the bulk of the leakage has
been localized in or just outside fifteen. To each of these
premises he goes with the knowledge that a portion of the total
leakage of 2000 gallons an hour is almost certainly there, and that
it must be found, which is a very different thing from visiting
three or four hundred houses, in not one of which he has any
particular reason to expect to find leakage. Even when he enters a
house with previous knowledge that there is leakage, its discovery
may be difficult. It is often hidden, sometimes underground, and
may only be brought to light by excavation. In these cases, without
some such system of localization, the leakage might go on for years
or for ever. There are many and obvious variations of the system.
That described requires a diagram revolving once in a few hours,
otherwise the time scale will be too close; but the ordinary
diagram revolving once in 24 hours is often used quite effectively
in night inspections by only closing those stop-cocks which are
actually passing water. This method was also first introduced in
Liverpool. The night inspector carries with him a stethoscope, often
consisting merely of his steel turning-rod, with which he sounds the whole of the outside
stop-cocks, but only closes those through which the -sound of water
is heard. An experienced man, or even a boy, if selected as
possessing the necessary faculty (which is sometimes very strongly
marked), can detect the smallest dribble when the stop - cock is so
far closed as to restrict the orifice. Similar examinations by means
of the stop-valves on the mains are also made, and it often happens
that the residual leakage (400 gallons an hour in the last case)
recorded on the diagram, but not shut off by the house stop -
cocks, is mentioned by the inspector as an " outside waste," and
localized as having been heard at a stop-cock and traced by sounding the pavement to a particular
position under a particular street. All leakages found on private
property are duly notified to the water tenant in the usual way, and subsequent
examinations are made to ascertain if such notices have been
attended to. If this work is properly organized, nearly the whole
of the leakage so detected is suppressed within a month. A record
of the constantly fluctuating so-called " night readings " in a
large town is most interesting and instructive. If, for example, in
the case of a hundred such districts we watch the result of leaving them alone, a gradual
growth of leakage common to most of the districts, but not to all,
is observed, while here and there a sudden increase occurs, often
doubling or trebling the total supply to the district. Upon the
original installa - tion of the system in any town, the rate of
leakage and consequent total supply to the different districts is
found;to vary greatly, and in some districts it is usually many
times as great per head as in others. An obvious and fruitful
extension of the method is to employ the inspectors only in those
districts which, for the time being, promise the most useful
results.

In many European cities the supply of water, even for domestic
purposes, is given through ordinary water meters, and paid for,
according to the meter record, much in the same manner as a supply
of gas or electricity. By the adoption of this method
great reductions in the quantity of water used and wasted are in
some cases effected, and the water tenant pays for the leakage or
waste he permits to take place, as well as for the water he uses.
The system, however, does not assist in the detection of the
leakage which inevitably occurs between the reservoir and the
consumer's meter; thus the whole of the mains, joints and ferrules
connecting the service pipes with the mains, and the greater parts
of the service pipes, are still exposed to leakage without any
compensating return to the water authority. But the worst evil of
the system, and one which must always prevent its introduction into
the United Kingdom, is the circumstance that it treats water as an
article of commerce, to be paid for according to the quantity
taken. In the organization of the best municipal water undertakings
in the United Kingdom the free use of water is encouraged, and it
is only the leakage or occasional improper employment of the water
that the water authority seeks, and that successfully, to suppress.
The objection to the insanitary effect of the meter-payment system
has, in some places, been sought to be removed by providing a fixed
quantity of water, assumed to be sufficient, as the supply for a
fixed minimum payment, and by using the meter records simply for
the purpose of determining what additional payment, if any, becomes
due from the water tenant. Clearly, if the excesses are frequent,
the limit must be too low; if infrequent, all the physical and
administrative complication involved in the system is employed to
very little purpose.

The question of the distribution of water, rightly considered,
resolves itself into a question of delivering water to the water
tenant, without leakage on the way, and of securing that the
fittings employed by the water tenant shall be such as to afford an
ample and ready supply at all times of the day and night without
leakage and without any unnecessary facilities for waste. If these
conditions are complied with, it is probable that the total rate of
supply will not exceed, even if it reaches, the rate necessary in
any system, not being an oppressive and insanitary system, by which
the water is paid for according to the quantity used. (G. F.
D.)